Hi babowa,

The General Pane of the Mail Preferences has the Option to set the App (or not). (I believe it is greyed out if there is not another Option (mine lists Outlook). ).
Messages has a similar one.
Firefox has one for the Browser side as it were.

(ergo it is likely you can set it in the preferred app if it is a separate app)
That said there does not appear to be an option to de-select it so that it does not start up.
Internet Accounts may list Mail as a potential option.

This AIM Screen Name could be used for emails and therefore added to Mail if I wished
The second blurred one is actually and @mac.com name used as a Valid AIM Screen Name (but Mail is not active there either).
Links on sites such as this are designed to open the listed mail app.

I am not sure if or how that might be made to go to a mail page/tab in a browser or if it can be how you select which service.

9:23 pm Friday; September 30, 2016
iMac 2.5Ghz i5 2011 (El Capitan)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone and an iPad (2)