Ask HN: E-mail providers other than Google Apps?

Our company currently hosts about 100 clients on a cPanel/WHM setup.

Due to challenges in regard to spam and security, we want to get our clients' e-mail accounts off of our server an onto a dedicated e-mail hosting service. Some of our clients already use Google Apps for Mail and it works well for them, however, I am wondering if there is a solution that would make the transfer of e-mail accounts from our server easier than Google Apps seems to allow.

Any recommendations for a good IMAP-based e-mail hosting service?