I'm using Rackspace Email [ http://www.rackspace.com/email-hosting/ ] for several domains. You don't need any other Rackspace services, but there is a 10 mailbox minimum. There are two options: exchange and rackspace. The rackspace option is cheaper ($2/mailbox) and used to be called webmail.us - it support IMAP and push without issue. The exchange one is more expensive ($10/mo) but is a full exchange backend for your end users who need outlook. Both services are pay on demand, no contract. The other added bonus: Rackspace will take all the "I can't send email" calls for you. I routinely tell my end users to just call Rackspace for email issues instead of me. That alone makes it worth it's weight in gold.