Rackspace also allows you to run exchange mailboxes alongside IMAP if you have a few users who are still tied to outlook.
Additionally it seemed like they were chasing the smaller accounts. Whenever I consulted the help/documentation it focused on the converting the 1-5 user shops into paid accounts. There wasn't many resources for someone who walked in expecting to pay $2k/month. I had to dig for details. Eventually I created a paid account just to test all the details.
Contrast that with rackspace where everything worked smoothly out of the box. Any limitations were spelled out clearly. In general it felt like all my needs were anticipated. In two years with them I've opened maybe three tickets all solved exceedingly fast.