(via rackerhacker.com)
Adjusting sendmail queue time / lifetime
By default, sendmail will keep items in the queue for up to 5 days. If you want to make this something shorter, like 3 days, you can adjust the following in /etc/mail/sendmail.mc:
define(`confTO_QUEUERETURN', `3d')dnl
...
sendmail submission queuereturn timeout
The same "define(`confTO_QUEUERETURN', `3d')dnl" can also be used in "submit.mc" for the submission client mail queue to reduce the queuereturn timeout.