Very good point regarding "design like it's 1995". Too many people use too much CSS leading to their emails looking horrible (esp. with regards to positioning/tableless CSS).
If I can add an additional tip, those using a template engine should take the extra time to design a different template per major ISP. The tiny bit of extra work WILL pay dividends if it means your mail looks as good as possible for each client.
ie. Design a slightly different (test the rendering of course) template for AOL Mail versus Yahoo Mail versus Hotmail versus Blackberry (mobile) addresses, etc... then a generic template for anyone using their own domain name email.
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