Not a big post today.  Just a solution to a problem I ran into earlier.   This may appear self explanatory if you assume disk space issue, as in this case the error is actually not referring to mailbox size.

PROBLEM

Feb  8 04:02:06 lhs sendmail[4105]: l18924AB004105: low on space (SMTP-DAEMON needs 0 bytes + 100 blocks in /var/spool/clientmqueue), max avail: 0
Feb  8 04:02:06 lhs sendmail[4105]: l18924AA004105: SYSERR(root): returntosender: cannot select queue for root

SOLUTION

Your /var folder / partition might be full. (Likewise, just your ‘/‘ root partition might be full if you have your entire system under a single partition.)  Use 'du -kh –max-depth=1' and df to determine which files are the culprits.  If you upgrade your linux distribution often, this might be due to yum files in 'packages' and 'headers' folders taking up a huge amount of space.  For this look into ‘/var/cache/yum‘.   One other thing to check is your '/var/spool/clientmqueue' folder in case you have lot's of stale messages there. You should also check out the ‘/tmp‘ and ‘/var/tmp‘ folders on your distribution for old files you may have sitting there.  You can also use a combination:

du -kh –max-depth=1

AND

ls -altriS|awk '{ print $6" "$10 }';

Within /var/spool or other large folders to determine where the size usage is highest.  To get an overall picture which partitions are taking up the most usage on your system, use df command.