Tracking mode MTA option

Tracking Information Storage
The  MTA option controls how  message tracking information is stored by the MTA. The default value of 0 disables storage of tracking information, while setting  to 1 enables storage using a  memcached server shared across the  deployment. The use of other values is currently restricted.

See also:
 * Message tracking and recall
 * Memcache MTA options
 * Message tracking MTA options