[DigitalSea]

When developing a HTML email it's best to have style tags at the top and not link to them using <link rel> because the way HTML email works and various clients, you have to bring the CSS inline for it to work properly across different email clients anyway.

It's a trial and error process. Build HTML emails using tables for as much as possible, you can't use background images just background colours, don't rely on floats or other common CSS properties (even using height or width in your CSS isn't as supported as well as you'd think) and the number one thing to remember is: the day a client complains it doesn't look good in Lotus Notes, you will want to throw your computer out the window and become a farmer because it's never going to work in Lotus Notes.

Always make sure your images are set to display block to fix issues with various email services like Windows Mail and Yahoo! Mail. If you need to set them side-by-side, add in another td inside of your table row.

Always make sure you write alternative text for images. Most email clients are set to not to display images in emails unless told otherwise. Another convenient feature not many know about is that you can style the alt text that shows. <img src="ourlogo.png" alt="Acme Company" style="color:#000;font-size:20px;" /> — doing this ensures even if someone without images still sees something that doesn't look like hideous empty squares.

With later versions of Outlook, Microsoft also stripped back support for various CSS attributes and HTML tags compared to previous version so you'll find you will encounter a lot of frustration with that as well.