exim is a mail transfer agent for Unix environments. exim follows the sendmail design model, and is highly configurable with many features.

Some of my favourite commands are:

exim -bp | exiqsumm
- Generates a summary about all of the messages currently in the queue, including total and time spent in the queue

exiwhat
- Shows what exim is currently doing

exim -Mrm <message-id>
- Remove a message from the queue

exiqgrep -z -i | xargs exim -Mrm
- Delete all frozen messages from the queue