MTA_SUBADDRESS

Specify a subaddress to use when generating a return address from a user name specified with the MTA_USER item code. The item_address and item_length fields specify the address and length of a text string containing the subaddress. The length of the string may not exceed ALFA_SIZE bytes. Only one subaddress may be specified per message.

The MTA_USER action must be used in conjunction with this item code.