Outlook incarnations as well as on Lotus Notes, AOL and the major online email services. It'll cost you though. Check your HTML One of the most useful best practices I've learned whilst doing HTML email and websites is to validate the HTML wherever possible. You can paste in a link and the W3C Validator or the Premailer will tell you where you're going wrong. Make sure there aren't any open tags. Some browsers may do a decent job of displaying invalid HTML pages but Outlook and other email clients won't be so forgiving. Use these tools with common sense as some errors will be OK with HTML emails. Don't get angry, get even Don't bang on the table, pull your hair out or write vitriolic attacks agains Microsoft in your Twitter feed like I did -- it won't get you anywhere. Instead have a cup of tea, breathe deeply and affirm "I will beat this. I will do this. I will finish this." When in doubt, print it out If your eyes are squinting at the computer screen, try printing out the HTML (presuming you're not short of paper or printer ink). When looking at the HTML on paper you can cross off the tab pairs which makes things easier to understand. Other HTML email "rules" You can't do everything in an HTML email that you can do on a webpage. It is important that your client understands this. Here are a few more do's and don'ts for HTML email: