GitHub User: Muralha
I've been working on email marketing for a while now and I have to say that Gmail is the 1st email client that I use to test my emails, but then I have to test in the painful Outlooks... because the majority of clients uses Outlook (Link).
Making emails "work" in Outlook 2007 & 2010 (Windows versions) is painful, because M$ switched the render engine from IE to Word (join your voice at Link) and the Word engine doesn't follow some web standards, so be ready for some headaches.
* be careful when sending a lot of emails, your email address can get blacklisted very fast
* use several email clients to test your emails
* forget CSS positions
* use a spam tester, to test your email's spam value
* add an unsubscribe link to your email, because it's mandatory (SPAM-CAN laws)
Some useful resources: