Blocklimit, linelimit, sourceblocklimit, noblocklimit, nolinelimit Channel Options

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Message size limits (blocklimit, linelimit, sourceblocklimit, noblocklimit, nolinelimit)

Although fragmentation may be used to break messages into smaller pieces automatically, it may also be appropriate in some cases to simply reject outright messages larger than some administratively defined limit, (e.g., so as to avoid service denial attacks on the system or individual mailboxes).

The blocklimit, linelimit and sourceblocklimit channel options are used to impose absolute size limits. Each of these options requires a single integer argument. blocklimit specifies the maximum number of blocks allowed in a message. The MTA will reject attempts to queue messages containing more blocks than this to the channel. The sourceblocklimit specifies the maximum number of blocks allowed in an incoming message. The MTA will reject attempts to submit a message containing more blocks than this to the channel. In other words, blocklimit applies to destination channels;

sourceblocklimitapplies to source channels. An MTA block is normally 1024 bytes; this can be changed with the block_size MTA option. linelimit specifies the maximum number of lines allowed in a message. The MTA will reject attempts to queue messages containing more than this number of lines to the channel. These limits can be imposed simultaneously if necessary.

Note that the line_limit and block_limit MTA options can be used to impose similar limits on all channels. These limits have the advantage that since they apply across all channels the MTA can make them known to mail clients via the SMTP SIZE extension prior to obtaining message recipient information. This simplifies the process of message rejection in some situations.

The nolinelimit and noblocklimit settings are the default and mean that no limits are imposed, other than any global limits imposed via the line_limit or block_limit MTA settings.