Others say this can happen when the queue gets clogged with messages.
Have a look at the answer here: http://serverfault.com/questions/615116/abnormal-sendmail-activity-using-up-the-server-memory-on-my-ubuntu-12-04-server
Nice thank you it's a good place to start!
I did the following:
with command
sendmail -O QueueSortOrder=none -Am -bp
Total requests: 136906
with command
sendmail -O QueueSortOrder=none -Ac -bp
Total requests: 549
Instruction
Stop Sendmail
service sendmail stop
rm /var/spool/mqueue/*
about error /bin/rm: Argument list too long use command: find . -name '*' | xargs rm
If you want to remove messages in waiting, use:
rm /var/spool/mqueue-client/*.
Restart Sendmail
service sendmail start