Via Using Emacs as Default Mailer on macOS, a tiny script to handle mailto: links.
From //Script Editor/, save following script as Application (MailOnEmacs.app). From Mail.app, Preferences -> Default email reader and chosse MailOnEmacs.app.
on open location myurl
tell application "Emacs" to activate
set text item delimiters to {":"}
do shell script "/path/to/emacsclient --eval '(browse-url-mail \"" & myurl & "\")'"
end open location