Hi Lanny,
You've posted this in the AppleWorks community, where the main topic is issues and techniques involving Apple's long discontinued productivity application AppleWorks.
Definitely not the 'right' location, but practically, a location whose current purpose seems to be seen as a 'junk drawer' in which to place things for which there is no apparent 'correct' location.
You wrote "This topic is about the Mail app, but there is no ability to select the Mail app in the topics list. "
Agreed.
Applications with no named community and which are bundled with the operating system,are generally considered to be 'system software', and should be directed to th community for the version of OS X or macOS installed on your computer.
That said, your "Please add…" request iis not one tht can be handled in this group of user to user communities. Adding capabilities can be handled only by Apple leadership an the development teams for the product to which the request ins directed.
For most Apple applications, Feature enhancement request can be sent via the "Provide 'application' Feedback menu item in the Appplication menu. For Mail, the Feedback forms can be found here: https://www.apple.com/feedback/mail.html
Select Feature request as the feedback type, and make your request.
Aside…
Personally, I'd be hesitant to use the requested feature, considering the number of email messages that don't require that action but do get sent to my Junk folder (without my having written a rule that wound send them there.
Regards,
Barry