I'm currently running MailEnable as my email server, it's ok but lacking. I'm looking for a better alternative and am wondering about Open Source. I'm a huge fan of the Smarter Mail, and will pay if it's the best I can get... But I have been learning that there is some good stuff out there in the Open Source community (IE: I've started using PFSense and I love it).
OS doesn't matter, though I would prefer Windows.
Does anyone have any suggestions?
I asked a similar question on Hacker News.
Open-source components (free):
Self-hosted solutions (full version is not free):
3rd-party-hosted solutions (full version is not free):
Take a look at Zimbra Collaboration Suite. They have FOSS (Open Source) and Network edition. Great AJAX web client interface. Overall great complete e-mail server.
For my personal domain I use hMailserver on Windows plus RoundCube on a Linux box for webmail. I can't see any reason RoundCube can't be run on Windows as well, if that's what you prefer.
hMailserver isn't Open Source but it is free. It's simplicity itself to install and configure. It provides SMTP, POP3 and IMAP. It includes built-in spam filtering, although I don't use that and therefore can't tell you how effective it is.
I'm not aware of many open source email server projects on Windows. There's the free hMailServer, but that seems a little spartan. Since you mentioned that you weren't afraid to pay for something, I'll mention a few closed-source offerings that rival Exchange in functionality and blow it away for price:
For Linux, you've got quite an array:
Just to have it in here, Postfix is quite popular in the Linux/UNIX world, but it only handles SMTP so it isn't a complete solution for what you want. Plus, as far as I know, there's no Windows version.
+1 for Postfix and Roundcube
Also look at ISPConfig if you're looking to run DNS/SQL/Web/FTP as well as mail, it puts everything in a nice web interface, although it is Linux based