I have used Rackspace mail for 5 or 6 years, since I gave up trying managing email servers for firm and my clients. All it took was one client getting a spamming virus, and all email from the IP address would get black listed.
I have found their email to be very reliable. The webmail aspect is not perfect, and some of my clients have run into occasional issues with it. I almost always use an IMAP client app and never have an issue. You can get an upgrade and enable full mobile sync using (I think) MS ActiveSync.