The SIZE keyword to the MAIL FROM: command states how big an incoming message is in bytes.^[39] 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:

^[39] There is no guarantee that the size specified is accurate.

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).

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=).

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.

The MaxMessageSize option is not safe. If specified from the command line, it can cause sendmail to relinquish its special privileges.