What doctype to use when generating HTML mail

I'm working on a CMS website which sends newsletters based off an HTML page (and with a conversion to plain text as a multipart/alternative option). I wonder what would be the best practise for choosing doctype.

The design for these pages are of course simplified from the real website, and getting the right doctype for the page as such is trivial - but then again: this newsletter page will be shown in various "browsers", from gmail to Outlook. Webbased email clients will have their own doctype already in place, and locally installed email clients will probably have different engines, probably with different requirements.

So, what - if any - doctype should be used in the HTML mail?