MTA_FRAGMENT_LINES

Specify the maximum number of message lines per message. If, when the message is enqueued, the number of message lines exceeds this limit, then the message will be fragmented into smaller messages, each fragment with no more than the specified number of lines. The individual fragments are MIME compliant messages that use MIME’s message/partial content type. MIME compliant mailers or user agents that receive the fragments may automatically reassemble the fragmented message. (MTA channels must be marked with the defragment keyword in order for automatic message reassembly to occur.)

The item_length field specifies the maximum number of message lines per message or message fragment. By default, no limit is imposed.