I'm looking to replace our aging mail server and I've decided that I want to change the SMTP server software we use. We currently run exim. Over the years I've worked with a number of other MTAs (namely Postfix and Sendmail), and in my opinion, they all suck as far as configuration is concerned: they're all arcane, fiddly and inelegant, at least compared to the other mission-critical daemons I have to work with on a daily basis (Apache, MySQL, Dovecot, etc).
Our mail server does not handle much volume (we use MessageLabs to filter inbound mail: I am sure you will appreciate the pleasure that can be had from blocking access to tcp/25 on your firewall) so I don't need something that's high performance. Spam and anti-virus are also not too important, again thanks to MessageLabs. What I really want is a simple, terse configuration syntax that doesn't require arcane knowledge to read and write. Something that can accept mail for n domain names, and deliver to our IMAP server according to a simple mapping.