Specify the address of a procedure that will return, one line at a time, data for the message body. Each line of input obtained from the procedure will be treated using the current access mode and encoded using the current encoding as specified by MTA_MODE_ and MTA_ENC_ item codes. Note, however, that the block access mode will not be applied to input procedures.
The item_address field specifies the address of the procedure to invoke. The item_length field is ignored.
The calling format that must be used by the procedure is given in Message Headers and Content.