alternatives

The alternatives utility automates managing multiple programs with the same function.

For instance if you want to use qmail instead of the default sendmail, install qmail and then issue:

alternatives --install /usr/sbin/sendmail mta /var/qmail/bin/sendmail 50 --slave /usr/lib/sendmail mta-sendmail /var/qmail/bin/sendmail

There are more slave links to explore, but I think it works for the most part.

Then run alternatives --config mta to verify that qmail is the preferred choice.

To go back you can simply run the later command again and make a new choice.