In my application, I\'m currently using mailto: links to open email compose dialogs. This is terrible for web mail users, since it typically causes Outlook or Apple Mail to
Regarding which other web mail clients to support it depends a lot on the users. In Germany for example you should add GMX and web.de to the list.
Some searching led to this as the proper format for AOL Mail links:
http://webmail.aol.com/Mail/ComposeMessage.aspx?to=TO&subject=SUBJECT&body=BODY&cc=CC&bcc=BCC
Source: http://dev.aol.com/api/openmail/webmail (under 'External Invocation of Web Mail'), and some Google searching to find the parameters.
Once you have these 4 covered, I think you've got essentially all of the webmail clients covered.
The example google URL above returns a google error.
The February 2014 post to thread 2583928 recommends replacing view=cm&fs=1&tf=1
with &v=b&cs=wh
Try these updated ones
For outlook
https://outlook.live.com/mail/0/deeplink/compose?to=your@email.com
For AOL
https://mail.aol.com/webmail-std/en-us/composemessage?to=your@email.com
But I should warn you that, prefilled body if at all contains line feed(new line) character encoded in URL, that line feed won't take effect at present.