This has definitely been posted before and I still suggest http://www.fastmail.fm. They've been around for a while and are always upgrading and updating their services.
If you want something enterprise-y, I'd suggest Rackspace either hosted Exchange or their IMAP solution (less expensive).
The key is to recognize that when you're getting something for free that you are not the customer. Fastmail provides a great product at a reasonable price. Spam protection and support are both top notch.
If you want something enterprise-y, I'd suggest Rackspace either hosted Exchange or their IMAP solution (less expensive).