xref: /netbsd-src/external/ibm-public/postfix/dist/RELEASE_NOTES-2.3 (revision 4b004442778f1201b2161e87fd65ba87aae6601a)

The stable Postfix release is called postfix-2.3.x where 2=major
release number, 3=minor release number, x=patchlevel.  The stable
release never changes except for patches that address bugs or
emergencies. Patches change the patchlevel and the release date.

New features are developed in snapshot releases. These are called
postfix-2.4-yyyymmdd where yyyymmdd is the release date (yyyy=year,
mm=month, dd=day).  Patches are never issued for snapshot releases;
instead, a new snapshot is released.

The mail_release_date configuration parameter (format: yyyymmdd)
specifies the release date of a stable release or snapshot release.

Critical notes
--------------

See RELEASE_NOTES_2.2 if you upgrade from Postfix 2.1 or earlier.

Some Postfix internal protocols have changed.  You need to "postfix
reload" or restart Postfix, otherwise many servers will log warning
messages like "unexpected attribute xxx" or "problem talking to
service yyy", and mail will not be delivered.

The Sendmail-compatible Milter support introduces three new queue
file record types. As long as you leave this feature turned off,
you can still go back to Postfix version 2.2 without losing mail
that was received by Postfix 2.3.

Major changes - DNS lookups
---------------------------

[Incompat 20050726] Name server replies that contain a malformed
hostname are now flagged as permanent errors instead of transient
errors. This change works around a questionable proposal to use
syntactically invalid hostnames in MX records.

Major changes - DSN
-------------------

[Feature 20050615] DSN support as described in RFC 3461 .. RFC 3464.
This gives senders control over successful and failed delivery
notifications.  DSN involves extra parameters to the SMTP "MAIL
FROM" and "RCPT TO" commands, as well as extra Postfix sendmail
command line options for mail submission.

See DSN_README for details. Some implementation notes can be found
in implementation-notes/DSN.

[Incompat 20050615] The new DSN support conflicts with VERP support.
For Sendmail compatibility, Postfix now uses the sendmail -V command
line option for DSN.  To request VERP style delivery, you must now
specify -XV instead of -V.  The Postfix sendmail command will
recognize if you try to use -V for VERP-style delivery.  It will
usually do the right thing, and remind you of the new syntax.

[Incompat 20050828] Postfix no longer sends DSN SUCCESS notification
after virtual alias expansions when the cleanup server rejects the
content or size of mail that was submitted with the Postfix sendmail
command, mail that was forwarded with the local(8) delivery agent,
or mail that was re-queued with "postsuper -r". Since all the
recipients are reported as failed, the SUCCESS notification seems
redundant.

Major changes - LMTP client
---------------------------

See the "SASL authentication" and "TLS" sections for changes related
to SASL authentication and TLS support, respectively.

[Feature 20051208] The SMTP client now implements the LMTP protocol.
Most but not all smtp_xxx parameters now have an lmtp_xxx equivalent.
This means there are lot of new LMTP features, including support
for TLS and for the shared connection cache. See the "SMTP client"
section for details.

[Incompat 20051208] The LMTP client now reports the server as
"myhostname[/path/name]".  With the real server hostname in delivery
status reports, the information will be more useful.

Major changes - Milter support
------------------------------

[Feature 20060515] Milter (mail filter) application support,
compatible with Sendmail version 8.13.6 and earlier. This allows
you to run a large number of plug-ins to reject unwanted mail, and
to sign mail with for example domain keys. All Milter functions are
implemented except replacing the message body, which will be added
later. Milters are before-queue filters, so they don't change the
queue ID.

See the MILTER_README document for a discussion of how to use Milter
support with Postfix, and limitations of the current implementation.

The Sendmail-compatible Milter support introduces three new queue
file record types. As long as you leave this feature turned off,
you can still go back to Postfix version 2.2 without losing mail
that was received by Postfix 2.3.

[Incompat 20060515] Milter support introduces new logfile event
types: milter-reject, milter-discard and milter-hold, that identify
actions from Milter applications. This may affect logfile processing
software.

Major changes - SASL authentication
-----------------------------------

[Feature 20051220] Plug-in support for SASL authentication in the
SMTP server and in the SMTP/LMTP client. With this, Postfix can
support multiple SASL implementations without source code patches.
Some distributors may even make SASL support a run-time linking
option, just like they already do with Postfix lookup tables.

Hints and tips for plug-in developers are in the xsasl/README file.

For backwards compatibility the default plug-in type is Cyrus SASL,
so everything should behave like it did before. Some error messages
are slightly different, but these are generally improvements.

The "postconf -a" command shows what plug-in implementations are
available for the SMTP server, and "postconf -A" does the same for
the SMTP/LMTP client.  Plug-in implementations are selected with
the smtpd_sasl_type, smtp_sasl_type and lmtp_sasl_type configuration
parameters.

Other new configuration parameters are smtpd_sasl_path, smtp_sasl_path
and lmtp_sasl_path. These are better left alone; they are introduced
for the convenience of other SASL implementations.

[Feature 20051222] Dovecot SASL support (SMTP server only). Details
can be found in the SASL_README document.

[Incompat 20051220] The Postfix-with-Cyrus-SASL build procedure has
changed. You now need to specify -DUSE_CYRUS_SASL in addition to
-DUSE_SASL_AUTH or else you end up without any Cyrus SASL support.
The error messages are:

    unsupported SASL server implementation: cyrus
    unsupported SASL client implementation: cyrus

[Feature 20051125] This snapshot adds support for sender-dependent
ISP accounts.

- Sender-dependent smarthost lookup tables.  The maps are searched
  with the sender address and with the sender @domain.  The result
  overrides the global relayhost setting, but otherwise has identical
  behavior. See the postconf(5) manual page for more details.

  Example:
    /etc/postfix/main.cf:
	sender_dependent_relayhost_maps = hash:/etc/postfix/sender_relay

- Sender-dependent SASL authentication support. This disables SMTP
  connection caching to ensure that mail from different senders
  will use the correct authentication credentials. The SMTP SASL
  password file is first searched by sender address, and then by
  the remote domain and hostname as usual.

  Example:
    /etc/postfix/main.cf:
	smtp_sasl_auth_enable = yes
	smtp_sender_dependent_authentication = yes
	smtp_sasl_password_maps = hash:/etc/postfix/sasl_pass

[Incompat 20060707] The SMTP/LMTP client now defers delivery when
a SASL password exists but the server does not announce support for
SASL authentication.  This can happen with servers that announce
SASL support only when TLS is turned on.  When an opportunistic TLS
handshake fails, Postfix >= 2.3 retries delivery in plaintext, and
the remote server rejects mail from the unauthenticated client.
Specify "smtp_sasl_auth_enforce = no" to deliver mail anyway.

Major changes - SMTP client
---------------------------

See the "SASL authentication" and "TLS" sections for changes related
to SASL authentication and TLS support, respectively.

[Feature 20051208] The SMTP client now implements the LMTP protocol.
Most but not all smtp_xxx parameters now have an lmtp_xxx equivalent.
This means there are lot of new LMTP features, including support
for TLS and for the shared connection cache.

[Incompat 20060112] The Postfix SMTP/LMTP client by default no
longer allows DNS CNAME records to override the server hostname
that is used for logging, SASL password lookup, TLS policy selection
and TLS server certificate verification. Specify
"smtp_cname_overrides_servername = yes" to get the old behavior.

[Incompat 20060103] The Postfix SMTP/LMTP client no longer defers
mail delivery when it receives a malformed SMTP server reply in a
session with command pipelining.  When helpful warnings are enabled,
it will suggest that command pipelining be disabled for the affected
destination.

[Incompat 20051208] The fallback_relay feature is renamed to
smtp_fallback_relay, to make clear that the combined SMTP/LMTP
client uses this setting only for SMTP deliveries. The old name
still works.

[Incompat 20051106] The relay=... logging has changed and now
includes the remote SMTP server port number as hostname[hostaddr]:port.

[Incompat 20051026] The smtp_connection_cache_reuse_limit parameter
(which limits the number of deliveries per SMTP connection) is
replaced by the new smtp_connection_reuse_time_limit parameter (the
time after which a connection is no longer stored into the connection
cache).

[Feature 20051026] This snapshot addresses a performance stability
problem with remote SMTP servers. The problem is not specific to
Postfix: it can happen when any MTA sends large amounts of SMTP
email to a site that has multiple MX hosts. The insight that led
to the solution, as well as an initial implementation, are due to
Victor Duchovni.

The problem starts when one of a set of MX hosts becomes slower
than the rest.  Even though SMTP clients connect to fast and slow
MX hosts with equal probability, the slow MX host ends up with more
simultaneous inbound connections than the faster MX hosts, because
the slow MX host needs more time to serve each client request.

The slow MX host becomes a connection attractor.  If one MX host
becomes N times slower than the rest, it dominates mail delivery
latency unless there are more than N fast MX hosts to counter the
effect. And if the number of MX hosts is smaller than N, the mail
delivery latency becomes effectively that of the slowest MX host
divided by the total number of MX hosts.

The solution uses connection caching in a way that differs from
Postfix 2.2.  By limiting the amount of time during which a connection
can be used repeatedly (instead of limiting the number of deliveries
over that connection), Postfix not only restores fairness in the
distribution of simultaneous connections across a set of MX hosts,
it also favors deliveries over connections that perform well, which
is exactly what we want.

The smtp_connection_reuse_time_limit feature implements the connection
reuse time limit as discussed above.  It limits the amount of time
after which an SMTP connection is no longer stored into the connection
cache. The default limit, 300s, can result in a huge number of
deliveries over a single connection.

This solution will be complete when Postfix logging is updated to
include information about the number of times that a connection was
used. This information is needed to diagnose inter-operability
problems with servers that exhibit bugs when they receive multiple
messages over the same connection.

[Incompat 20050627] The Postfix SMTP client no longer applies the
smtp_mx_session_limit to non-permanent errors during the TCP, SMTP,
HELO or TLS handshake.  Previous versions did that only with TCP
and SMTP handshake errors.

[Incompat 20050622] The Postfix SMTP client by default limits the
number of MX server addresses to smtp_mx_address_limit=5. Previously
this limit was disabled by default. The new limit prevents Postfix
from spending lots of time trying to connect to lots of bogus MX
servers.

Major changes - SMTP server
---------------------------

See the "SASL authentication" and "TLS" sections for changes related
to SASL authentication and TLS support, respectively.

[Feature 20051222] To accept the non-compliant user@ipaddress form,
specify "resolve_numeric_domain = yes".  Postfix will deliver the
mail to user@[ipaddress] instead.

[Incompat 20051202] The Postfix SMTP server now refuses to receive
mail from the network if it isn't running with postfix mail_owner
privileges.  This prevents surprises when, for example, "sendmail
-bs" is configured to run as root from xinetd.

[Incompat 20051121] Although the permit_mx_backup feature still
accepts mail for authorized destinations (see permit_mx_backup for
definition), with all other destinations it now requires that the
local MTA is listed as non-primary MX server. This prevents mail
loop problems when someone points their primary MX record at a
Postfix system.

[Feature 20051011] Optional suppression of remote SMTP client
hostname lookup and hostname verification. Specify "smtpd_peername_lookup
= no" to eliminate DNS lookup latencies, but do so only under extreme
conditions, as it makes Postfix logging less informative.

[Feature 20050724] SMTPD Access control based on the existence of
an address->name mapping, with reject_unknown_reverse_client_hostname.
There is no corresponding access table lookup feature, because the
name is not validated in any way (except that it has proper syntax).

Several confusing SMTPD access restrictions were renamed:

    reject_unknown_client -> reject_unknown_client_hostname,
    reject_unknown_hostname -> reject_unknown_helo_hostname,
    reject_invalid_hostname -> reject_invalid_helo_hostname,
    reject_non_fqdn_hostname -> reject_non_fqdn_helo_hostname.

The old names are still recognized and documented.

Major changes - TLS
-------------------

Major revisions were made to Postfix TLS support; see TLS_README
for the details. For backwards compatibility, the old TLS policy
user interface will be kept intact for a few releases so that sites
can upgrade Postfix without being forced to use a different TLS
policy mechanism.

[Feature 20060614] New concept: TLS security levels ("none", "may",
"encrypt", "verify" or "secure") in the Postfix SMTP client.  You
can specify the TLS security level via the smtp_tls_security_level
parameter.  This is more convenient than controlling TLS with the
multiple smtp_use_tls, smtp_enforce_tls, and smtp_tls_enforce_peername,
parameters.

[Feature 20060709] TLS security levels ("none", "may", "encrypt")
in the Postfix SMTP server. You specify the security level with the
smtpd_tls_security_level parameter. This overrides the multiple
smtpd_use_tls and smtpd_enforce_tls parameters.  When one of the
unimplemented "verify" or "secure" levels is specified, the Postfix
SMTP server logs a warning and uses "encrypt" instead.

[Feature 20060123] A new per-site TLS policy mechanism for the
Postfix SMTP client that supports the new TLS security levels,
and that eliminates DNS spoofing attacks more effectively.

[Feature 20060626] Both the Postfix SMTP client and server can be
configured without a client or server certificate. An SMTP server
without certificate can use only anonymous ciphers, and will not
inter-operate with most clients.

The Postfix SMTP server supports anonymous ciphers when 1) no client
certificates are requested or required, and 2) the administrator
has not excluded the "aNULL" OpenSSL cipher type with the
smtpd_tls_exclude_ciphers parameter.

The Postfix SMTP client supports anonymous ciphers when 1) no server
certificate is required and 2) the administrator has not excluded
the "aNULL" OpenSSL cipher type with the smtp_tls_exclude_ciphers
parameter.

[Incompat 20060707] The SMTPD policy client now encodes the
ccert_subject and ccert_issuer attributes as xtext. Some characters
are represented by +XX, where XX is the two-digit hexadecimal
representation of the character value.

[Feature 20060614] The smtpd_tls_protocols parameter restricts the
list of TLS protocols supported by the SMTP server.  This is
recommended for use with MSA configurations only.  It should not
be used with MX hosts that receive mail from the Internet, as it
reduces inter-operability.

[Incompat 20060614] The smtp_tls_cipherlist parameter only applies
when TLS is mandatory.  It is ignored with opportunistic TLS sessions.

[Incompat 20060614] At (lmtp|smtp|smtpd)_tls_loglevel >= 2, Postfix
now also logs TLS session cache activity. Use level 2 and higher
for debugging only; use levels 0 or 1 as production settings.

[Incompat 20060207] The Postfix SMTP server no longer complains
when TLS support is not compiled in while permit_tls_clientcerts,
permit_tls_all_clientcerts, or check_ccert_access are specified in
main.cf. These features now are effectively ignored.  However, the
reject_plaintext_session feature is not ignored and will reject
plain-text mail.

[Feature 20060123] Some obscure behavior was eliminated from the
smtp_tls_per_site feature, without changes to the user interface.
Some Postfix internals had to be re-structured for the new TLS
policy mechanism; for this, smtp_tls_per_site had to be re-implemented.
The obscure behavior was found during compatibility testing.

[Feature 20051011] Optional protection against SMTP clients that
hammer the server with too many new (i.e. uncached) SMTP-over-TLS
sessions. Cached sessions are much less expensive in terms of CPU
cycles. Use the smtpd_client_new_tls_session_rate_limit parameter
to specify a limit that is at least the inbound client concurrency
limit, or else you may deny legitimate service requests.

Major changes - VERP
--------------------

[Incompat 20050615] The new DSN support conflicts with VERP support.
For Sendmail compatibility, Postfix now uses the sendmail -V command
line option for DSN.  In order to request VERP style delivery, you
must now specify -XV instead of -V.  The Postfix sendmail command
will recognize if you try to use -V for VERP-style delivery.  It
will do the right thing and will remind you of the new syntax.

Major changes - XCLIENT and XFORWARD
------------------------------------

[Incompat 20060611] The SMTP server XCLIENT implementation has
changed. The SMTP server now resets state to the initial server
greeting stage, immediately before the EHLO/HELO greeting.  This
was needed to correctly simulate the effect of connection-level
access restrictions.  Without this change, XCLIENT would not work
at all with Milter applications.

[Incompat 20060611] The SMTP server XCLIENT and XFORWARD commands
now expect that attributes are xtext encoded (RFC 1891). For backwards
compatibility they will also accept unencoded attribute values. The
XFORWARD client code in the SMTP client and in the SMTPD_PROXY
client now always encode attribute values. This change will have a
visible effect only for malformed hostname and helo parameter values.

For more details, see the XCLIENT_README and XFORWARD_README
documents.

Major changes - address manipulation
------------------------------------

[Incompat 20060123] Postfix now preserves uppercase information
while mapping addresses with canonical, virtual, relocated or generic
maps; this happens even with $number substitutions in regular
expression maps. However, the local(8) and virtual(8) delivery
agents still fold addresses to lower case.

As a side effect, Postfix now also does a better job at being case
insensitive where it should be, for example while searching per-host
TLS policies or SASL passwords.

By default, Postfix now folds the search string to lowercase only
with tables that have fixed-case lookup fields such as btree:,
hash:, dbm:, ldap:, or *sql:. The search string is no longer case
folded with tables whose lookup fields can match both upper or lower
case, such as regexp:, pcre:, or cidr:.

For safety reasons, Postfix no longer allows $number substitution
in regexp: or pcre: transport tables or per-sender relayhost tables.

Major changes - bounce message templates
----------------------------------------

[Feature 20051113] Configurable bounce messages, based on a format
that was developed by Nicolas Riendeau. The file with templates is
specified with the bounce_template_file parameter. Details are in
the bounce(5) manual page, and examples of the built-in templates
can be found in $config_directory/bounce.cf.default. The template
for the default bounce message looks like this:

    failure_template = <<EOF
    Charset: us-ascii
    From: MAILER-DAEMON (Mail Delivery System)
    Subject: Undelivered Mail Returned to Sender
    Postmaster-Subject: Postmaster Copy: Undelivered Mail

    This is the $mail_name program at host $myhostname.

    I'm sorry to have to inform you that your message could not
    be delivered to one or more recipients. It's attached below.

    For further assistance, please send mail to <postmaster>

    If you do so, please include this problem report. You can
    delete your own text from the attached returned message.

                            The $mail_name program
    EOF

Major changes - built-in filters
--------------------------------

[Feature 20050828] Configurable filters to reject or remove unwanted
characters in email content. The message_reject_characters and
message_strip_characters parameters understand the usual C-like
escape sequences: \a \b \f \n \r \t \v \ddd (up to three octal
digits) and \\.

[Incompat 20050828] When a header/body_checks rule or when
message_reject_characters rejects mail that was submitted with the
Postfix sendmail command (or re-queued with "postsuper -r"), the
returned message is now limited to just the message headers, to
avoid the risk of exposure to harmful content in the message body
or attachments.

Major changes - database support
--------------------------------

[Incompat 20060611] The PostgreSQL client was updated after the
PostgreSQL developers made major database API changes in response
to SQL injection problems.  This breaks support for PGSQL versions
prior to 8.1.4, 8.0.8, 7.4.13, and 7.3.15. Support for these requires
major code changes which are not possible in the time that is left
for completing the Postfix 2.3 stable release.

Major changes - enhanced status codes
-------------------------------------

[Feature 20050328] This release introduces support for RFC 3463
enhanced status codes.  For example, status code 5.1.1 means
"recipient unknown". Postfix recognizes enhanced status codes in
remote server replies, generates enhanced status codes while handling
email, and reports enhanced status codes in non-delivery notifications.
This improves the user experience with mail clients that translate
enhanced status codes into text in the user's own language.

You can, but don't have to, specify RFC 3463 enhanced status codes
in the output from commands that receive mail from a pipe. If a
command terminates with non-zero exit status, and an enhanced status
code is present at the beginning of the command output, then that
status code takes precedence over the non-zero exit status.

You can, but don't have to, specify RFC 3463 enhanced status codes
in Postfix access maps, header/body_checks REJECT actions, or in
RBL replies.  For example:

    REJECT 5.7.1 You can't go here from there

The status 5.7.1 means "no authorization, message refused", and is
the default for access maps, header/body_checks REJECT actions, and
for RBL replies.

[Feature 20050328] If you specify your own enhanced status code,
the Postfix SMTP server will automatically change a leading '5'
digit (hard error) into '4' where appropriate.  This is needed, for
example, with soft_bounce=yes.

[Feature 20050510] This release improves usability of enhanced
status codes in Postfix access tables, RBL reply templates and in
transport maps that use the error(8) delivery agent.

- When the SMTP server rejects a sender address, it transforms a
  recipient DSN status (e.g., 4.1.1-4.1.6) into the corresponding
  sender DSN status, and vice versa.

- When the SMTP server rejects non-address information (such as the
  HELO command parameter or the client hostname/address), it
  transforms a sender or recipient DSN status into a generic
  non-address DSN status (e.g., 4.0.0).

These transformations are needed when the same access table or RBL
reply template are used for client, helo, sender, or recipient
restrictions; or when the same error(8) mailer information is used
for both senders and recipients.

Major changes - local alias expansion
-------------------------------------

[Incompat 20051011] The Postfix local(8) delivery agent no longer
updates its idea of the Delivered-To: address while it expands
aliases or .forward files. With deeply nested aliases or .forward
files, this can greatly reduce the number of queue files and cleanup
process instances. To get the earlier behavior, specify
"frozen_delivered_to = no".

The frozen_delivered_to feature can help to alleviate a long-standing
problem with multiple deliveries to recipients that are listed
multiple times in a hierarchy of nested aliases. For this to work,
only the top-level alias should have an owner- alias, and none of
the subordinate aliases.

Major changes - logging
-----------------------

[Incompat 20060515] Milter support introduces new logfile event
types: milter-reject, milter-discard and milter-hold, that identify
actions from Milter applications. This may affect logfile processing
software.

[Incompat 20051106] The relay=... logging has changed and now
includes the remote SMTP server port number as hostname[hostaddr]:port.

[Incompat 20060112] The Postfix SMTP/LMTP client by default no
longer allows DNS CNAME records to override the server hostname
that is used for logging, SASL password lookup, TLS policy selection
and TLS server certificate verification. Specify
"smtp_cname_overrides_servername = yes" to get the old behavior.

[Incompat 20051105] All delay logging now has sub-second resolution,
including the over-all "delay=nnn" logging.  A patch is available
for pflogsumm (pflogsumm-conn-delays-dsn-patch). The qshape script
has been updated (auxiliary/qshape/qshape.pl).

[Feature 20051103] This release makes a beginning with a series of
new attributes in Postfix logfile records.

- Better insight into the nature of performance bottle necks, with
  detailed logging of delays in various stages of message delivery.
  Postfix logs additional delay information as "delays=a/b/c/d"
  where a=time before queue manager, including message transmission;
  b=time in queue manager; c=connection setup time including DNS,
  HELO and TLS; d=message transmission time.

- Logging of the connection reuse count when SMTP connections are
  used for more than one message delivery.  This information is
  needed because Postfix can now reuse connections hundreds of times
  or more. Logging of the connection reuse count can help to diagnose
  inter-operability problems with servers that suffer from memory
  leaks or other resource leaks.

At this point the Postfix logging for a recipient looks like this:

    Nov  3 16:04:31 myname postfix/smtp[30840]: 19B6B2900FE:
    to=<wietse@test.example.com>, orig_to=<wietse@test>,
    relay=mail.example.com[1.2.3.4], conn_use=2, delay=0,
    delays=0/0.01/0.05/0.1, dsn=2.0.0, status=sent (250 2.0.0 Ok)

The following two logfile fields may or may not be present:

    orig_to     This is omitted when the address did not change.
    conn_use    This is omitted when a connection is used once.

[Incompat 20050503] The format of some "warning:" messages in the
maillog has changed so that they are easier to sort:

- The logging now talks about "access table", instead of using three
  different expressions "access table", "access map" and "SMTPD
  access map" for the same thing.

- "non-SMTP command" is now logged BEFORE the client name/address
  and the offending client input, instead of at the end.

[Incompat 20050328] The logging format has changed. Postfix delivery
agents now log the RFC 3463 enhanced status code as "dsn=x.y.z"
where y and z can be up to three digits each.

[Incompat 20051208] The LMTP client now reports the server as
"myhostname[/path/name]".  With the real server hostname in delivery
status reports, the information will be more useful.

Major changes - performance
---------------------------

[Incompat 20051105] All delay logging now has sub-second resolution,
including the over-all "delay=nnn" logging.  A patch is available
for pflogsumm (pflogsumm-conn-delays-dsn-patch). The qshape script
has been updated (auxiliary/qshape/qshape.pl).

[Incompat 20050622] The Postfix SMTP client by default limits the
number of MX server addresses to smtp_mx_address_limit=5. Previously
this limit was disabled by default. The new limit prevents Postfix
from spending lots of time trying to connect to lots of bogus MX
servers.

[Feature 20051026] This snapshot addresses a performance stability
problem with remote SMTP servers. The problem is not specific to
Postfix: it can happen when any MTA sends large amounts of SMTP
email to a site that has multiple MX hosts. The insight that led
to the solution, as well as an initial implementation, are due to
Victor Duchovni.

The problem starts when one of a set of MX hosts becomes slower
than the rest.  Even though SMTP clients connect to fast and slow
MX hosts with equal probability, the slow MX host ends up with more
simultaneous inbound connections than the faster MX hosts, because
the slow MX host needs more time to serve each client request.

The slow MX host becomes a connection attractor.  If one MX host
becomes N times slower than the rest, it dominates mail delivery
latency unless there are more than N fast MX hosts to counter the
effect. And if the number of MX hosts is smaller than N, the mail
delivery latency becomes effectively that of the slowest MX host
divided by the total number of MX hosts.

The solution uses connection caching in a way that differs from
Postfix 2.2.  By limiting the amount of time during which a connection
can be used repeatedly (instead of limiting the number of deliveries
over that connection), Postfix not only restores fairness in the
distribution of simultaneous connections across a set of MX hosts,
it also favors deliveries over connections that perform well, which
is exactly what we want.

The smtp_connection_reuse_time_limit feature implements the connection
reuse time limit as discussed above.  It limits the amount of time
after which an SMTP connection is no longer stored into the connection
cache. The default limit, 300s, can result in a huge number of
deliveries over a single connection.

This solution will be complete when Postfix logging is updated to
include information about the number of times that a connection was
used. This information is needed to diagnose inter-operability
problems with servers that exhibit bugs when they receive multiple
messages over the same connection.

[Feature 20051011] Optional protection against SMTP clients that
hammer the server with too many new (i.e. uncached) SMTP-over-TLS
sessions. Cached sessions are much less expensive in terms of CPU
cycles. Use the smtpd_client_new_tls_session_rate_limit parameter
to specify a limit that is at least the inbound client concurrency
limit, or else you may deny legitimate service requests.

[Feature 20051011] Optional suppression of remote SMTP client
hostname lookup and hostname verification. Specify "smtpd_peername_lookup
= no" to eliminate DNS lookup latencies, but do so only under extreme
conditions, as it makes Postfix logging less informative.

Major changes - portability
---------------------------

[Incompat 20050716] Internal interfaces have changed; this may break
third-party patches because the types of function arguments and of
result values have changed.  The types of buffer lengths and offsets
were changed from "int" or "unsigned int" (32 bit on 32-bit and
LP64 systems) to "ssize_t" or "size_t" (64 bit on LP64 systems, 32
bit on 32-bit systems).

This change makes no difference in Postfix behavior on 32-bit
systems. On LP64 systems, however, this change not only eliminates
some obscure portability bugs, it also eliminates unnecessary
conversions between 32/64 bit integer types, because many system
library routines take "(s)size_t" arguments or return "(s)size_t"
values.

This change may break software on LP64 systems 1) when Postfix is
linked with pre-compiled code that was compiled with old Postfix
interface definitions and 2) when compiling Postfix source that was
modified by a third-party patch: incorrect code will be generated
when the patch passes the wrong integer argument type in contexts
that disable automatic argument type conversions.  Examples of such
contexts are formatting with printf-like arguments, and invoking
functions that write Postfix request or reply attributes across
inter-process communication channels.  Unfortunately, gcc reports
"(unsigned) int" versus "(s)size_t" format string argument mis-matches
only on LP64 systems.

Major changes - safety
----------------------

[Incompat 20051121] Although the permit_mx_backup feature still
accepts mail for authorized destinations (see permit_mx_backup for
definition), with all other destinations it now requires that the
local MTA is listed as non-primary MX. This prevents mail loop
problems when someone points the primary MX record at a Postfix
system.

[Incompat 20051011] The Postfix local(8) delivery agent no longer
updates its idea of the Delivered-To: address while it expands
aliases or .forward files. With deeply nested aliases or .forward
files, this can greatly reduce the number of queue files and cleanup
process instances. To get the earlier behavior, specify
"frozen_delivered_to = no".

The frozen_delivered_to feature can help to alleviate a long-standing
problem with multiple deliveries to recipients that are listed
multiple times in a hierarchy of nested aliases. For this to work,
only the top-level alias should have an owner- alias, and none of
the subordinate aliases.

[Incompat 20050828] When a header/body_checks rule or when
message_reject_characters rejects mail that was submitted with the
Postfix sendmail command (or re-queued with "postsuper -r"), the
returned message is now limited to just the message headers, to
avoid the risk of exposure to harmful content in the message body
or attachments.

[Incompat 20051202] The Postfix SMTP server now refuses to receive
mail from the network if it isn't running with postfix mail_owner
privileges.  This prevents surprises when, for example, "sendmail
-bs" is configured to run as root from xinetd.

[Incompat 20060123] For safety reasons, Postfix no longer allows
$number substitution in regexp: or pcre: transport tables or
per-sender relayhost tables.

[Incompat 20060112] The Postfix SMTP/LMTP client by default no
longer allows DNS CNAME records to override the server hostname
that is used for logging, SASL password lookup, TLS policy selection
and TLS server certificate verification. Specify
"smtp_cname_overrides_servername = yes" to get the old behavior.