Although fragmentation can automatically break messages into smaller pieces, it is appropriate in some cases to reject messages larger than some administratively-defined limit, (for example, to avoid service denial attacks). The blocklimit and linelimit keywords are used to impose absolute size limits. Each of these keywords must be followed by a single integer value.
blocklimit specifies the maximum number of blocks allowed in a message. IMTA rejects attempts to queue messages containing more blocks than this to the channel. An IMTA block is normally 1024 bytes; this can be changed with the BLOCK_SIZE option in the IMTA option file.
linelimit specifies the maximum number of lines allowed in a message. IMTA rejects attempts to queue messages containing more than this number of lines to the channel. These two limit blocklimit and linelimit, can be imposed simultaneously, if necessary.
IMTA options LINE_LIMIT and BLOCK_LIMIT can be used to impose similar limits on all channels. These limits have the advantage that they apply across all channels. Therefore, IMTA's servers can make them known to mail clients prior to obtaining message recipient information. This simplifies the process of message rejection in some protocols.