Which/what mail solution for 3000 users?

Hi all,

So I have this problem, we have about 3000 users who need e-mail. It's a split between windows and Mac users and I need a mail server for them all. I'd like webmail as well as IMAP/POP. Antivirus and AntiSpam would also be critical.

What do people here suggest? Are their any appliances which will do the job? or am I looking at running an XServer with some software??

Any suggestions welcome.

Thanks
--Zed