Use comment strings MTA option

Miscellaneous mapping table MTA options:  (bitmask)
(New in MS 6.1.) The  MTA  option takes a bit encoded integer argument. If bit 0 (value 1) of this option is set, then the familiar source-channel&#x7c;destination-channel&#x7c; prefix will be included in    mapping table probes. See the  and   channel options, and  RFC 822 comments strings and personal name modification  for more discussion of the    mapping table.

See also:
 * Miscellaneous mapping table MTA options
 * COMMENT_STRINGS mapping table
 * commentmap Option
 * sourcecommentmap Option