Tweaking Configuration Options

There are a large number of configuration options that don't normally need to be changed. However, if you feel you need to tweak them, you can define the following M4 variables. This list is shown in four columns: the name you define, the default value for that definition, the option or macro that is affected (either Ox for an option or Dx for a macro), and a brief description. Greater detail of the semantics can be found in the Installation and Operations Guide.

Some options are likely to be deprecated in future versions -- that is, the option is only included to provide back-compatibility. These are marked with "*".

Remember that these options are M4 variables, and hence may need to be quoted. In particular, arguments with commas will usually have to be ``double quoted, like this phrase'' to avoid having the comma confuse things. This is common for alias file definitions and for the read timeout.

See also the description of OSTYPE for some parameters that can be tweaked (generally pathnames to mailers).

ClientPortOptions and DaemonPortOptions are special cases since multiple clients/daemons can be defined. This can be done via

Note that multiple CLIENT_OPTIONS() commands (and therefore multiple ClientPortOptions settings) are allowed in order to give settings for each protocol family (e.g., one for Family=inet and one for Family=inet6). A restriction placed on one family only affects outgoing connections on that particular family.

If DAEMON_OPTIONS is not used, then the default is

If you use one DAEMON_OPTIONS macro, it will alter the parameters of the first of these. The second will still be defaulted; it represents a "Message Submission Agent" (MSA) as defined by RFC 2476 (see below). To turn off the default definition for the MSA, use FEATURE(` no_default_msa') (see also FEATURES). If you use additional DAEMON_OPTIONS macros, they will add additional daemons.

Example 1:
To change the port for the SMTP listener, while still using the MSA default, use

Example 2:
To change the port for the MSA daemon, while still using the default SMTP port, use

Note that if the first of those DAEMON_OPTIONS lines were omitted, then there would be no listener on the standard SMTP port.

Example 3:
To listen on both IPv4 and IPv6 interfaces, use

A "Message Submission Agent" still uses all of the same rulesets for processing the message (and therefore still allows message rejection via the check_* rulesets). In accordance with the RFC, the MSA will ensure that all domains in the envelope are fully qualified if the message is relayed to another MTA. It will also enforce the normal address syntax rules and log error messages. Additionally, by using the M=a modifier you can require authentication before messages are accepted by the MSA.

Notice: Do NOT use the 'a' modifier on a public accessible MTA!

Finally, the M=E modifier shown above disables ETRN as required by RFC 2476.

Mail filters can be defined using the INPUT_MAIL_FILTER() and MAIL_FILTER() commands:

The INPUT_MAIL_FILTER() command causes the filter(s) to be called in the same order they were specified by also setting confINPUT_MAIL_FILTERS. A filter can be defined without adding it to the input filter list by using MAIL_FILTER() instead of INPUT_MAIL_FILTER() in your .mc file. Alternatively, you can reset the list of filters and their order by setting confINPUT_MAIL_FILTERS option after all INPUT_MAIL_FILTER() commands in your .mc file.