The ESMTP SIZE keyword to the MAIL FROM: command tells V8 sendmail how big an incoming message is in bytes. If the SIZE keyword is not specified, sendmail assumes that the incoming message is zero bytes in size. In either case it calls an internal routine to see whether enough space is available in the queue to accept the message. Unless sendmail is told otherwise, it assumes it can use 100% of the disk space in the queue. If SIZE bytes will overfill the queue disk, sendmail prints the following error and rejects the mail message:

Insufficient disk space; try again later

Note that the SIZE keyword (if received) is just an estimate that allows oversized mail to be rejected early in the ESMTP dialog. V8 sendmail still properly diagnoses out-of-space conditions when it actually reads the message.

If using 100% of the disk space is unacceptable, you can use the MinFreeBlocks option, the forms of which follow, to reserve space for other kinds of files:

O MinFreeBlocks=minblocks                    configuration file (V8.7 and later) 
-OMinFreeBlocks=minblocks                    command line (V8.7 and later) 
define(`confMIN_FREE_BLOCKS',minblocks)      mc configuration (V8.7 and later) 
Obminblocks/maxsize                          configuration file (deprecated) 
-obminblocks/maxsize                         command line (deprecated) 

Here, minblocks is of type numeric and is the number of disk blocks you wish to reserve. If minblocks is missing or negative, or if the entire option is omitted, no blocks are reserved. For the V8.6 form of the b option a slash is required to separate minblocks from maxsize (maxsize is described under the MaxMessageSize option, MaxMessageSize). The default when configuring with the mc method is 100.

Note that minblocks minimum blocks are reserved only for the ESMTP SIZE keyword to the MAIL FROM: command. No check is made for any other kind of queuing to reserve space. Consequently, you should reserve a sufficient number of blocks to satisfy your normal queuing needs.

The MinFreeBlocks option is safe. Even if it is specified from the command line, sendmail retains its special privileges.