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8<title>Postfix Configuration Parameters </title>
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16<h1><img src="postfix-logo.jpg" width="203" height="98" alt="">Postfix Configuration Parameters </h1>
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18<hr>
19
20<h2> Postfix main.cf file format </h2>
21
22<p> The Postfix main.cf configuration file specifies a very small
23subset of all the parameters that control the operation of the
24Postfix mail system. Parameters not explicitly specified are left
25at their default values. </p>
26
27<p> The general format of the main.cf file is as follows: </p>
28
29<ul>
30
31<li> <p> Each logical line is in the form "parameter = value".
32Whitespace around the "=" is ignored, as is whitespace at the end
33of a logical line. </p>
34
35<li> <p> Empty lines and whitespace-only lines are ignored, as are
36lines whose first non-whitespace character is a `#'. </p>
37
38<li> <p> A logical line starts with non-whitespace text. A line
39that starts with whitespace continues a logical line. </p>
40
41<li> <p> A parameter value may refer to other parameters. </p>
42
43<ul>
44
45<li> <p> The expressions "$name", "${name}" or "$(name)" are
46recursively replaced by the value of the named parameter. </p>
47
48<li> <p> The expression "${name?value}" expands to "value" when
49"$name" is non-empty. This form is supported with Postfix version
502.2 and later. </p>
51
52<li> <p> The expression "${name:value}" expands to "value" when
53"$name" is empty. This form is supported with Postfix version 2.2
54and later.  </p>
55
56<li> <p> Specify "$$" to produce a single "$" character. </p>
57
58</ul>
59
60<li> <p> When the same parameter is defined multiple times, only
61the last instance is remembered. </p>
62
63<li> <p> Otherwise, the order of main.cf parameter definitions does
64not matter. </p>
65
66</ul>
67
68<p> The remainder of this document is a description of all Postfix
69configuration parameters. Default values are shown after the
70parameter name in parentheses, and can be looked up with the
71"<b>postconf -d</b>" command. </p>
72
73<p> Note: this is not an invitation to make changes to Postfix
74configuration parameters. Unnecessary changes are likely to impair
75the operation of the mail system.  </p>
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77<dl>
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