Could you guys with experience of mail servers tell me what you'd see as the best option for an integrated mail/calendaring solution with public folders for a Windows network (even if the server is non Win)?

We've had Exchange (5.5), and Scalix with Outlook. Both worked as far as our users were concerned, but neither was particularly easy for me to manage. One of the reasons we're moving to Exchange is that we now have a few users who want mail through their phones and I've been told this is is easier via Exchange.

I like Thunderbird a lot, but it's sufficiently different from Outlook for me to have to go through it with all my users, likewise (I guess) any other client, although Outlook 2007 is hugely different from 2000 (our current version)...