Background story: Neither Outlook 2007 nor Outlook 2010 support background images. Include a background color in a 100% width table wrapped around your email as a fallback to any background images you want to use.
Keep it in line: Style sheets may be nice and convenient, but they can be a one way ticket to trouble for email clients. Put your styles inline to make sure they don’t get stripped out entirely.
Simplicity is key: Remember, the more you have going on in your email, the more there is to go wrong with the rendering. Besides, clarity of design is an essential part of effective communication, right? Oops, sorry – different blog post.
Take a survey: Test your emails on as many email clients as possible before sending. That way you’ll have a good idea of how it will behave under different send circumstances, so you can adjust and make changes as needed.
Avoid shortcuts: Don’t use abbreviations in your HTML, don’t use float tags, don’t omit the dimensions of your images – and don’t skip testing!
Know your audience Are you a business-to-business type? Keep in mind you have to deal with Lotus Notes. Are you a vacation destination? Consider optimizing for mobile email. Collect as much information as possible about how people interact with your direct digital marketing so you know what your primary concerns may be.
For more on email marketing, check out these posts.