[mgbmtl]

Zimbra has packages for Ubuntu and CentOS which are really easy to install and upgrade.

Pros: It provides a good enough webmail, a good (collaborative) calendar, LDAP integration, etc. Configuring SPF, DKIM and https/smtps/imaps was pretty easy (good official wiki docs).

Cons: Since it's a very "industry ready" kind of project, with lots of corporations using it, so the community forums can be very hit or miss. "Community" (free software) downloads are harder to find on their website, and the binary package is a bit annoying to keep up to date (does not use apt/yum).

Overall, I find it sufficiently turn-key and it "just works", while still being based on free software. I also feel comfortable recommending it to clients who want to move away from MS Exchange or Google Apps, since there are lots of Zimbra commercial providers out there who can support it.

ps: Zimbra/mail is not part of my main business focus, I only manage our mail server because I don't like depending on a 3rd-party for something as critical as mail.

[rebootthesystem]

The minimum system requirements for Zimbra feel excessive.

[mgbmtl]

Fair point. The web UI and (iirc) imap/pop daemons are Java based. The smtpd is postfix. It also uses MySQL for some of its storage, and OpenLDAP for authentication. On the other hand, it scales well.