I found emailonacid.com today (beta, currently free†) - have only played with it a little but so far so good. It simulates the following clients:
- AOL 9
- Entourage 2004 & 2008
- Gmail
- Hotmail
- Windows Live Mail
- Windows Mail
- Mac Mail
- Outlook 2003 & 2007
- Thunderbird 2, 3 & Beta
- Yahoo Classic / Yahoo Mail
The very helpful thing about this service is it tells you what code is not supported in which client.
†Edit: Not free anymore, but provides a 7 day free trial.
