You must always include your physical (mailing) address. Most of our templates include a built-in tag that automatically pulls your address from your account settings. In some cases, you may need to manually code in your address. The federal Can-Spam law regulating commercial email requires a physical address somewhere within your mailing. A P.O. box will do in a pinch.
We recommend building your mailing's HTML with an HTML editing program like Dreamweaver. HTML editing programs help identify errors in your HTML design, as well as give you live previews of what you're building, while you're building it. Hand coding can be done, of course, but an HTML editor will help to simplify the design process. HTML editors like Taco for OSX or Coffee Cup for Windows can be a valuable alternative at a competitive price, while free online editors such as CSSDesk are also available.
Make sure to test your mailings before sending to your contacts. There is no global standard for HTML and CSS in email, so email clients such as Outlook, Gmail, Yahoo, etc., will vary in how they render your code. We want your designs to look consistently fabulous across each client, so testing is key. Set up addresses at several domains, add these addresses to your test group, and test away. For even more comprehensive testing, you can turn to online services like Litmus and Email on Acid.
And, remember, you can test for factors other than formatting. Set up split tests to compare different subject lines, sending times and RVSP names. These kinds of tests will help you determine what your contacts respond well to and when they're most likely to be receptive to your message.
It's important to remember that designing HTML for email is different from designing HTML for a webpage or print mailing. While there are a number of key similarities, there are many more key differences.