Create Cross-Client HTML E-mails – Premailer
HTML e-mails need some special attention when coding them.
We should be using inline CSS and absolute paths, many modern elements are not supported (table-usage is preferred), etc.
Premailer is a free web-based tool (an optional bookmarklet exists) that can help you create “more compatible” HTML e-mails by checking all above and even more.
Additionally, it creates a plain text version and checks the CSS properties against e-mail client capabilities.