sendmail, 4th Edition

MaxMessageSize

Maximum incoming ESMTP message size V8.7 and later

The SIZE keyword to the MAIL From: command states how big an incoming message is in bytes.^[396] If the SIZE keyword is not specified, sendmail makes no assumptions about the incoming message’s size. V8 sendmail can reject a message at this point if it is larger than a definable maximum message size:

Message size exceeds fixed maximum message size (max)

Here, max is the maximum acceptable size in bytes. Ordinarily, there is no maximum. If you want to define one, you can do so with the MaxMessageSize option:

O MaxMessageSize=maxsize                 ← configuration file (V8.7 and later)
-OMaxMessageSize=maxsize                 ← command line (V8.7 and later)
define(`confMAX_MESSAGE_SIZE',maxsize)   ← mc configuration (V8.7 and later)
Obminblocks/maxsize                      ← configuration file (deprecated)
-obminblocks/maxsize                     ← command line (deprecated)

If maxsize is omitted or if this entire option is omitted, the default is 0 (for unlimited message sizes). For the mc configuration the default is 0 (unlimited). Note that the old b option could also set the minimum blocks free (see MinFreeBlocks on page 1057).

This limit on message size is enforced during the SMTP dialog. Later, after a delivery agent has been selected, further limitations can be imposed by using the M= delivery agent equate (see M= on page 746).

The size of the message is also checked after the message is received (after receipt of the SMTP final DATA-dot) and will be rejected if it is too large at that time. ...