[mschuster91]

> Outlook in particular is a pain point

That depends on the environment. I assume you're talking about Outlook as a frontend for an enterprise Exchange setup?

If your mail server admin configures IMAP and SMTP correctly, it's a breeze to get it set up (you will need SSL though). Use "DOMAIN\user.name" as username together with your AD password (and for team/shared mailboxes, use DOMAIN\user.name\mailbox.name, where mailbox.name is the cn or sAMAccountName attribute of the mailbox AD entry). Thunderbird can handle everyday emailing as well as responding to calendar events that way; I'm not sure about integrating "real" calendar stuff. Auto-completion of mail addresses must be done separately in Thunderbird, you'll need the AD structure information (root DN, plus the DN of your AD account so you can log in).

What you'll miss is forwarding rules access, but that can be done via webmail if the need arises.