The conversion channel performs arbitrary body-part-by-body-part conversions on messages flowing through the MTA. Any subset of MTA message traffic can be selected for conversion and any set of programs or command procedures can be used to perform conversion processing. (The MTA's native conversion facilities are fairly limited, so the ability to call external converters is crucial.) A special "database" (stored as the
conversions MTA option in Unified Configuration, or in the
conversions file in legacy configuration) is consulted to choose an appropriate conversion for each body part.
For instance, third party document converters or virus scanning software may be hooked in for automatic execution via the conversion channel. Or sites may develop their own custom applications to hook in via the conversion channel.
Because the conversion channel is intended for "intermediate" processing of messages, the MTA has a special sort of routing available for it, whereby messages are routed without affecting the recipient address(es); see the
CONVERSIONS mapping table. This special sort of routing used to route messages through the conversion channel without modification to the actual addresses can also be used for other purposes: to route through third party channel programs, or to route out to third party spam/virus SMTP hosts (that will then relay messages back to the MTA). See the discussion of alternate channel routing via the CONVERSIONS mapping.