Well Sendmail gets a bad rap but it is the die-hard of the MTA world. Most distro's install Sendmail so that it just works right out of the box - no config mucking needed, just add DNS.
Postfix is lauded because of its security, but recently I've personally
discovered that on a heavy use system the latest Sendmail versions are
faster and handle the large loads with less resources. That's important
to me because of the consulting work I do. The Postfix configuration
files are much saner than Sendmails and that's fairly important to folks
who just want to play with an MTA.
Exim is very secure and also operates well under load, almost as well as Sendmail. It's not as secure as Postfix, and it's configuration files are bit more wacky, but it's extremely powerful and makes a good replacement for Sendmail. If I was going to dump Sendmail, this would be my choice for a replacement.
Qmail.... Well, it's good. Its fast. It does a bit of everything. It's not *quite* OpenSource, and the key developer is known for being an arrogant piss-ant. If it weren't for that excess baggage I would say give it a try.
Those are the big four used out in industry (available on Linux). There are hundreds more, but these four get most of the action, and deservedly so.
Don't forget Courier. It's a MTA, as well as a MDA. I've never used it, but it's supposed to be fast, easy to configure, and secure. But I've never used it in any capacity. Postfix is my MTA of choice, but I don't run the kinds of volumes that Jon mentions. (If I did, I'd move Spam Assassin and Cyrus IMAP to different boxes).
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