We are going the send our newsletters in HTML format soon. I have found a lot of information (like on Litmus) about which e-mail program and web mailer understands which CSS property. But the only think I couldn't figure out is which DOCTYPE we should use.
Our mass mailing tool always overwrites the DOCTYPE with HTML 4.0 Transitional:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
What would you say we should take to be compatible with most of the e-mail programs and web mailer? We don't use a lot of HTML and now special new CSS properties.
My research suggests that you're probably better off not using a doctype at all. Most clients ignore it, so leave it out and save a bit of weight.