At $work, the official email solution is Outlook on Windows, connected to an Exchange server. That's problematic for people with Linux on their desktop machine.
The Exchange server supports IMAP, and e-mail works fairly well using the usual suspects, e.g. Thunderbird. It also provides the web mail interface, which is fairly crap unless you use IE. (Any other favorite e-mail clients?)
The biggest problem is the Outlook Calendar. I still have found no viable Linux client that can replace it.