sendmail + cyrus works for me. old, tried, reliable, isn't a constantly moving target, and works just fine with T-bird as a mail client. if you need MORE than that, maybe evolution...
I've never figured out why *ANYONE* would *WANT* Exchange, considering how many viruses I've seen passed by it (in the past, homonym-pun deliberate) along with bloatiness and occasional _INCOMPATIBILITY_ with normal IMAP clients (like T-bird).
A 'used to' company I did work for on site had Exchange on a W2k3 server for corporate mail. Yukk. I did all of my work in either FreeBSD or Linux, and refused to use Virus Outbreak [aka MS Outlook] as a mail client. I also wanted to clone my IMAP directories on the Linux and BSD boxen. 'Tedious' was an understatement, until people stopped sending me exchange links [knowing I'd never see the files anyway]. Dual-booting was NOT an option. It required having multiple physical computers, the ones I did work on, and "that windows box" that had the e-mail (and other nonsense) on it.
I forget all of the problems it created for me. Most of them were solved by using ONLY POP MAIL and not receiving any 'exchange links' or calendar nonsense or anything ELSE that required their hideous API to make use of.
anyway, if you don't need a license for a windows server, you'll save money. If some legacy "thing" was written using something SQL-server-proprietary [like stored procedures], which I've blatantly opposed every time I was in the loop on the decision, then you could still save money by running SQL Server on the Linux server, which could (for free) run your DNS, web services, repository, e-mail, document tracker, SAN, etc. and, oh by the way, the database, too.
And the really FUN thing about Linux: if you do a backup using a tarball, restoration onto a DIFFERENT HARD DRIVE is pretty much straightforward. Try that with a windows serer and a non-identical hard drive [so ghost backups aren't a possibility, let's say]. There are just SO many things that are GREATLY simplified when *NOT* using micro-shaft operating systems as the host.