I use Campaign Monitor to send out my monthly (or sometimes slightly faster than that) newsletter. Campaign Monitor is an excellent service, and I’ve been really pleased with what they offer, especially for the price. They recently turned me onto Fractal, an HTML/CSS email validation tool. It’s simple to use…Just take the HTML and CSS portion of the email you’re about to send, post it in the textarea on the Fractal home page, and submit the form. The result will be a list of all HTML tags and CSS properties that have no or partial support in desktop, Web-based, or mobile email clients (Lotus Notes 6 & 7 came up a lot for me; I didn’t really people still used Lotus Notes!). Then you can use that information to tweak the HTML email, to insure better reliability for viewers.