Specify the maximum number of blocks per message. If, when the message is enqueued, the message size exceeds this limit, then the message will be fragmented into smaller messages, each fragment no larger than the specified block size. 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 size of a block may vary from site to site. Sites can change this value from its default value of 1,024 bytes. Use the MTA SDK routine mtaBLOCK_SIZE to determine the size in bytes of a block.
The item_length field specifies the maximum block size per message or message fragment. By default, no limit is imposed.