HTML Emails can be a pain in the neck due to radically differing rendering of various clients. There are a number of web-based services which cater to designers or developers for testing the results in a dozen or so various clients (and versions of the clients) from Outlook to Gmail, and even Lotus Notes.

Note that all three provide screenshots only.

http://litmusapp.com/
Litmus offers several tiers of service, including a limited free testing, to unlimited team subscription for $200/month. It's the only service (that I'm aware of) that's only for testing, and not for entire campaigns.

http://www.mailchimp.com/
Anything named after a monkey is cool with me. A full-service campaign manager with 'Inbox Inspector' for checking out the results.

http://www.campaignmonitor.com
Another full-service campaign manager with Inbox screenshots.