Updated November 9, 2010 for the first time in over 3 years (again).

Fake MTA Debian Package (564 bytes) - Fills the MTA requiremnt for dpkg/apt/etc. I still get lots of downloads from qmail users who want to get rid of exim.
(FYI: I no longer run qmail and have switched to exim on all my servers. It's much more flexible than qmail and integrates much better with spam filters, antivirus, etc. Give it a try!)

If you have not already installed any mail tools you can remove exim with:
dpkg --purge exim4

Then you can install the fake-mta package (which will let you install other mail packages like courier imap with:
dpkg -i fake-mta.deb

If you have already went through the qmr install and want to remove exim without messing with your other mail packages, you can use:
dpkg --force-conflicts -i fake-mta.deb
dpkg --purge exim4

This will let you remove exim4 without taking the other mail packages with it. The --purge option removes any config files and directories otherwise left behind when using -r or --remove. Either method will work, --purge is just cleaner if you dont need the exim config files. Keep in mind you will need root privlages to install and remove packages with dpkg.