Sendmail is age-old mail transfer agent (MTA). We still use sendmail on Solaris boxes and all other web hosting (www) server to route mails via our master MTA.
Task: Linux/UNIX deliver old email
The command sendmail -q forces the mail queue to be sent. Use the command mailq to find out what's in the queue:
# sendmail -q
Following command will force sendmail to become verbose so that debugging turns into an easy job:
# sendmail -v -q
However, there is a catch. Sendmail would not process your queue if the system load were too high. You can configure these options in sendmail configuration file sendmail.cf and configure QueueLA option. Using this option you can configure load average at which Sendmail simply queues up new messages, this is a good tweaking and troubleshooting parameter.