The MTA Configuration File

The primary MTA configuration file is imta.cnf. By default, this file is found at msg_svr_base/config/imta.cnf. This file contains MTA channel definitions as well as the channel rewrite rules. The channel associated with a rewritten destination address becomes the destination channel. The system will typically work well using default imta.cnf.

This section provides a brief introduction to the MTA configuration file. For details about configuring the rewrite rules and channel definitions that make up the MTA configuration file, see Chapter 11, Configuring Rewrite Rules, and Chapter 12, Configuring Channel Definitions.

By modifying the MTA configuration file, you establish the channels in use at a site and establish which channels are responsible for which sorts of addresses via rewrite rules. The configuration file establishes the layout of the email system by specifying the transport methods available (channels) and the transport routes (rewrite rules) associating types of addresses with appropriate channels.

The configuration file consists of two parts: domain rewriting rules and channel definitions. The domain rewriting rules appear first in the file and are separated from the channel definitions by a blank line. The channel definitions are collectively referred to as the channel table. An individual channel definition forms a channel block.

The following example of an imta.cnf configuration file shows how rewrite rules are used to route messages to the proper channel. No domain names are used to keep things as simple as possible. The rewrite rules appear in the upper half of the configuration file followed by the channel definitions in the lower half of the configuration file.

! test.cnf - An example configuration file.   (1)!
! This is only an example of a configuration file. It serves
! no useful purpose and should not be used in a real system.
!
! Part I: Rewrite rules
a     $U@a-daemon           (2)
b     $U@b-daemon
c     $U%c@b-daemon
d     $U%d@a-daemon    
      (3)
! Part II: Channel definitions
l      (4)
local-host

a_channel defragment charset7 usascii      (5)
a-daemon

b_channel noreverse notices 1 2 3
b-daemon

</opt/SUNWmsgsr/msg-tango/table/internet.rules    (6)

The key items (labeled with boldface numbers, enclosed in parentheses) in the preceding configuration file are explained in the following list:

1. Exclamation points (!) are used to include comment lines. The exclamation point must appear in the first column. An exclamation point appearing anywhere else is interpreted as a literal exclamation point.

2. The rewrite rules appear in the first half of the configuration file. No blank lines can appear among the lines of rewrite rules. Lines with comments (beginning with an exclamation point in the first column) are permitted.

3. The first blank line to appear in the file signifies the end of the rewrite rules section and the start of the channel blocks. These definitions are collectively referred to as the channel host table, which defines the channels that the MTA can use and the names associated with each channel.

4. The first channel block to appear is usually the local or l channel. Blank lines then separate each channel block from one another. (An exception is the defaults channel, which can appear before the l channel).

5. A typical channel definition consists of a channel name (a_channel) some keywords which define the configuration of a channel (defragment charset7 usascii) and a routing system (a-daemon), which is also called a channel tag.

6. The contents of other files may be included in the configuration file. If a line is encountered with a less than sign (<) in column one, the rest of the line is treated as a file name; the file name should always be an absolute and full file path. The file is opened and its contents are spliced into the configuration file at that point. Include files may be nested up to three levels deep. Any files included in the configuration file must be world-readable just as the configuration file is world-readable.

Table 10–1 shows how some example addresses would be routed by the preceding configuration.

Refer to Rewrite Rules, Channel Definitions, and Chapter 11, Configuring Rewrite Rules” for more information on the MTA configuration file.

Whenever changes are made to the imta.cnf file, the MTA configuration must be recompiled. See Compiling the MTA Configuration.