I was courious as to what the most used MTA was in our group.
Here at the office we used to be qmail fans. And for the most part qmail is a nice MTA. Biggest problem is the setup. Documentation is a little sparse and when things break it takes a while to figure out why they broke.
Courier on the other hand is a little easier to setup and comes with just about everything you need (smtp, pop3, imap, webmail, webadmin, mailing list manager, etc). Documentation is better. When things break it is still a bit of a pain to figure out what went wrong.
We have switched to courier due to this. If you are running a larger higher volume installation qmail may be the better choice but for small multi domain stuff courier seems to handle everything gracefuly.
The cool part is with both you can add spam and virus filtering at the SMTP local delivery level. Requires a little more horse power for the mail server but it makes that transparent to the user.