SITUATION: 

 For HTML mail, the effect of DOCTYPE falls in 3 categories,
   with well known implementations in each category:

   A) DOCTYPE ignored, falling back to no-quirks, is implemented by
      * Web mail apps (because app itself is in no-quirks) Examples: 
        # Microsoft Outlook.com
        # Google Gmail
      * Mozilla Gaia of Firefox OS (because it’s a offline Webmail app?)

   B) DOCTYPE ignored, falling back to quirks mode. Implemented by:
      * Mozilla Thunderbird 
        (https://bugzilla.mozilla.org/show_bug.cgi?id=335499)
      * Microsoft Email app for Windows 8.1

   C) DOCTYPE treated same as in Web browsers:
      * Microsoft Windows Live Mail 2012 (latest release: 2014)
      * Microsoft Windows Mail,
      * Microsoft Outlook Express,
      * Apple Mail for OSX
      * Apple mail for iOS

PROPOSAL:

For HTML mail, the DOCTYPE should be ignored, falling back to no-quirks rendering. THus doctype should not be required. This should be said in new section somewhere in the spec.

COMMMENT:

 Mail is a "date stamped" format that is stored inside private mailboxes. Thus legacy issues are probably seldom.  For online webmail, the rendering mode probably almost always depends on the rendering mode of the Web app.

Thus, all in all, the only reliable way forward is to make all HTML mail use no-quirks mode and only no-quirks mode. HTML mail is the number one production equipment for HTML and we gotta make it so that mail applications by default and with ease can produce conforming HTML. For many Web authors, it would be a benefit if it this way became simpler to get the same rendering mode etc across email readers.

Please see bug 25232, which proposes a general approach for de-emphasizing the DOCTYPE and instead emphasizing the rendering mode.