The second part of an MTA configuration file contains the definitions for the channels themselves. 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. Each individual channel definition forms a “channel block.” Blocks are separated by single blank lines. Comments (but no blank lines) may appear inside a channel block. A channel block contains a list of keywords which define the configuration of a channel. These keywords are referred to as “channel keywords.” See Table 4–6 for more information.
The following imta.cnf file fragment displays a sample channel block:
[blank line] ! sample channel block channelname keyword1 keyword2 routing_system [blank line]
The routing_system is the host name associated with this channel. During the address rewriting process, the host part of the address is checked against the hostnames associated with the channels before any pattern matching in the rewrite rules. The only exception to this is that the $* and exact pattern match rewrite rules are checked first.