I've also found Litmus to be an incredible resource for cross-client test (Outlook vs Gmail vs Yahoo! vs ....). If you send a lot of emails as a part of your user retention strateg it's vital that you test this. You'll be amazed at how different they look and how a few hours of tweaking can optimize things across a large number of clients.
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If you're designing email HTML from scratch, Campaign Monitor's guide to CSS is incredibly helpful for knowing what to use and what to avoid.
http://www.campaignmonitor.com/css/
I've also found Litmus to be an incredible resource for cross-client test (Outlook vs Gmail vs Yahoo! vs ....). If you send a lot of emails as a part of your user retention strateg it's vital that you test this. You'll be amazed at how different they look and how a few hours of tweaking can optimize things across a large number of clients.
http://www.litmus.com
Also useful, automatic CSS inlining: http://premailer.dialect.ca/ & https://github.com/alexdunae/premailer/
I'll second Litmus. It's one of the few tools that I've really felt were worth paying for. It's just tons better than try to test emails ad-hoc.