Sendmail
From MediaWiki
How To Flush All Email Messages From The Sendmail Queue
Sometimes email messages can be queued up with the Mail Transport Agent: "sendmail". The following command line can be used to manually try to immediately flush out All emails queued up by "sendmail". Even though a "sendmail" service may be running, this command is Ok to use, it will only run once and then terminate.
Try To Flush The Entire sendmail Queue:
[root@probe-222 root]# /usr/local/sbin/sendmail -q -v