13.5.2 Selecting Traffic for Conversion Processing

Unlike other MTA channels, the conversion channel is not normally specified in an address or MTA rewrite rule. Instead, messages are sent through the conversion channel if they meet the criteria specified in the CONVERSIONS mapping table (the name of the mappings file is specified by the parameter IMTA_MAPPING_FILE in the imta_tailor file. The default is msg_srv_base/conversions). Entries to the table have the following format:

IN-CHAN=source-channel;OUT-CHAN=destination-channel;CONVERT Yes/No

As the MTA processes each message it probes the CONVERSIONS mapping table (if one is present). If the source-channel is the channel from which the message is coming and destination-channel is the channel to which the message is going, then the action following CONVERT is taken. Yes means the MTA diverts the message through the conversion channel on its way to its destination-channel. If no match is found, or if No was specified, the message will be queued to the regular destination channel.

If you route messages to the conversion channel using conversion mappings your other mappings should continue to work. However, if you use rewrite rules to route messages to the conversion channel, you may have to adjust your mappings to accommodate what you've done.

An address of the form user@conversion.localhostname or user@conversion will be routed through the conversion channel, regardless of the CONVERSIONS mapping table.

The following example routes all non-internal messages—messages originating from, or destined to, the Internet—through the conversion channel.

CONVERSIONS

   IN-CHAN=tcp_local;OUT-CHAN=*;CONVERT   Yes
   IN-CHAN=*;OUT-CHAN=tcp_local;CONVERT   Yes

The first line specifies that messages coming from the tcp_local channel will be processed. The second line specifies that messages going to the tcp_local channel will also be processed. The tcp_local channel handles all messages going to and coming from the Internet. Since the default is to not go through the conversion channel, any other messages won’t go through the conversion channel.

Note that this is a very basic table, and that it might not be sufficient for a site with a more customized configuration, for example, one using multiple outbound-to-the-Internet tcp_* channels, or using multiple inbound-from-the-Internet tcp_* channels.