The MinFreeBlocks option (See this section) defines the minimum number of disk blocks that must be reserved on the queue disk. If an incoming SMTP message will fill the disk beyond this minimum, the message is rejected.

The -d4.80 debugging switch^[1] traces the enoughspace( ) routine in conf.c. That routine examines the disk space and allows or disallows incoming mail:

^[1] No -d4.1 (a.k.a. -d4) information is available yet.

enoughspace: no threshold

This debugging output says that no limit was defined with the MinFreeBlocks option:

enoughspace: bavail=haveblocks need=needblocks

This debugging output shows that the number of blocks free (available) on the disk is haveblocks and that the number of blocks required by incoming mail is needblocks. Note that haveblocks will always be -1 if sendmail was compiled with SFS_TYPE set to SFS_NONE (SFS_TYPE):

enoughspace failure: min=minfreeblocks need=needblocks

If the required number of blocks (needblocks) exceeds the minimum reserved as defined by the MinFreeBlocks option (minfreeblocks), use of the disk is disallowed.