Some email systems or network transfers cannot handle messages that exceed certain size limits. The MTA provides facilities to impose such limits on a channel-by-channel basis. Messages larger than the set limits are automatically split (fragmented) into multiple, smaller messages. The content type used for such fragments is message/partial, and a unique ID parameter is added so that parts of the same message can be associated with one another and, possibly, be automatically reassembled by the receiving mailer.
The maxblocks and maxlines keywords are used to impose size limits beyond which automatic fragmentation are activated. Both of these keywords must be followed by a single integer value. The keyword maxblocks specifies the maximum number of blocks allowed in a message. An MTA block is normally 1024 bytes; this can be changed with the BLOCK_SIZE option in the MTA option file. The keyword maxlines specifies the maximum number of lines allowed in a message. These two limits can be imposed simultaneously if necessary.
Message headers are, to a certain extent, included in the size of a message. Because message headers cannot be split into multiple messages, and yet they themselves can exceed the specified size limits, a rather complex mechanism is used to account for message header sizes. This logic is controlled by the MAX_HEADER_BLOCK_USE and MAX_HEADER_LINE_USE options in the MTA option file.
MAX_HEADER_BLOCK_USE is used to specify a real number between 0 and 1. The default value is 0.5. A message's header is allowed to occupy this much of the total number of blocks a message can consume (specified by the maxblocks keyword). If the message header is larger, the MTA takes the product of MAX_HEADER_BLOCK_USE and maxblocks as the size of the header (the header size is taken to be the smaller of the actual header size and maxblocks) * MAX_HEADER_BLOCK_USE.
For example, if maxblocks is 10 and MAX_HEADER_BLOCK_USE is the default, 0.5, any message header larger than 5 blocks is treated as a 5-block header, and if the message is 5 or fewer blocks in size it is not fragmented. A value of 0 causes headers to be effectively ignored insofar as message-size limits are concerned.
A value of 1 allows headers to use up all of the size that's available. Each fragment always contains at least one message line, regardless of whether or not the limits are exceeded by this. MAX_HEADER_LINE_USE operates in a similar fashion in conjunction with the maxlines keyword.