1[The patent discussed in this document expired in 2019, without a 2request for extension. The owner of that patent can no longer sue for 3infringement. However, other patents may make similar claims. The text 4below may serve as an example for dealing with them.] 5 61. Disclaimer: This text is not an authoritative statement. If 7you are concerned about the implications of US patent 6,321,267, 8then you should give this text to your own lawyer and get their 9advice. 10 111.1 Postfix is an MTA that aims to be an alternative to the widely 12 used Sendmail MTA. Postfix is available as open source code 13 from http://www.postfix.org/. One of the features implemented 14 by Postfix is called "sender address verification". 15 161.2 US patent 6,321,267 (reference 4.1) describes a number of means 17 to stop junk email. One of the elements described in this 18 patent is called "active user testing". 19 201.3 Postfix "sender address verification" and US patent 6,321,267 21 "active user testing" are implemented by connecting to an MTA 22 that is responsible for the sender address. Specifically, both 23 use the SMTP RCPT command, and both infer the validity of the 24 address from the MTA's response. Reference 4.3 defines SMTP. 25 26===================================================================== 27 282. It is my understanding that the Postfix MTA's "sender address 29verification" does not infringe on US patent 6,321,267 for the 30following reasons: 31 322.1 There is prior art for US patent 6,321,267 "active user testing" 33 within the context of the Sendmail MTA. See item (3.1) below. 34 352.2 US patent 6,321,267 covers "active user testing" only in 36 combination with functions that the Postfix MTA does not 37 implement. See items (3.2) through (3.5) below. 38 39===================================================================== 40 413. Discussion of specific details of US patent 6,321,267, and their 42relevance with respect to the Postfix MTA. 43 443.1 Prior art. The "active user testing" method is described in 45 the paper "Selectively Rejecting SPAM Using Sendmail" by Robert 46 Harker (reference 4.2). The paper is cited as the first 47 reference in US patent 6,321,267, and was presented in October 48 1997. The patent was filed more than two years later, in November 49 1999. The paper says: 50 51 Bogus User Address 52 53 A desirable criterion for rejecting mail is to filter on 54 bogus user address. However, testing for a bad user address 55 is much harder because, short of sending a message to that 56 user address, there is no reliable way to check the validity 57 of the address. A simplistic test for a bad user address 58 might be to connect to the sender's SMTP server and use 59 either the SMTP VRFY or RCPT command to check the address. 60 If the server does local delivery of the message then this 61 would work well. 62 63 The prior art is about stopping junk mail with the Sendmail 64 MTA. It is my understanding that this prior art is equally 65 applicable to other MTAs, including the Postfix MTA (see items 66 1.1 and 2.2 above). 67 683.2 Combination of elements not implemented by the Postfix MTA. 69 Claim 1 of US patent 6,321,267 involves a combination of A) 70 determining whether the sending system is a dialup host, B) 71 determining whether the sending system is an open mail relay, 72 and C) active user testing. 73 74 Postfix does not implement elements A) and B) of claim 1. 75 Therefore, it is my understanding that the Postfix MTA does 76 not infringe on US patent 6,321,267 claim 1. 77 783.3 Combination of elements not implemented by the Postfix MTA. 79 Claim 52 of US patent 6,321,267 involves the combination of A) 80 a proxy filter and B) active user testing. 81 82 Postfix is an MTA, not a proxy, and does not implement element 83 A) of claim 52. Therefore, it is my understanding that the 84 Postfix MTA does not infringe on US patent 6,321,267 claim 52. 85 86 US patent 6,321,267 makes a clear distinction between proxies 87 and MTAs. 88 89 Figure 13 in US patent 6,321,267 shows how a proxy interacts 90 with a sending system and a local MTA. In the case of (sending 91 system, proxy, local MTA), the proxy assumes no responsibility 92 for delivery of the email message. The responsibility remains 93 with the sending system or passes directly to the local MTA. 94 95 Figure 4 in US patent 6,321,267 shows how a sending system 96 interacts with an intermediate MTA. In the case of (sending 97 system, intermediate MTA, local MTA), the intermediate MTA 98 assumes full responsibility for delivery of the email message. 99 100 Figure 2 in US patent 6,321,267 shows how a sending system 101 interacts with a local MTA. In the case of (sending system, 102 local MTA), the local MTA assumes full responsibility for 103 delivery of the email message. 104 1053.4 The other independent claims in US patent 6,321,267 involve 106 elements that the Postfix MTA does not implement, and do not 107 involve sender address verification. Therefore, it is my 108 understanding that the Postfix MTA does not infringe on these 109 claims in US patent 6,321,267. 110 1113.5 All dependent claims in US patent 6,321,267 depend on claims 112 that involve elements that the Postfix MTA does not implement. 113 Therefore, it is my understanding that the Postfix MTA does 114 not infringe on these claims in US patent 6,321,267. 115 1164.References: 117 1184.1 Albert L. Donaldson, "Method and apparatus for filtering junk 119 email", US patent 6,321,267. Filing date: November 23, 1999. 120 http://www.uspto.gov/ 121 1224.2 Robert Harker, "Selectively Rejecting SPAM Using Sendmail", 123 Proceedings of the Eleventh Systems Administration Conference 124 (LISA '97), San Diego, California, Oct. 1997, pp. 205-220. 125 http://www.usenix.org/publications/library/proceedings/lisa97/ 126 full_papers/22.harker/22.pdf 127 1284.3 Jonathan B. Postel, "Simple Mail Transfer Protocol", August 129 1982. http://www.ietf.org/rfc.html 130