Is it possible to activate whichever mail client is connected by default to a mailto link in applescript?
Whatever opens MAILTO:// links is set from within the Mail.app itself Go to Mail > Preferences > General and the top pop-up menu lets you set any installed application that the OS recognizes as being able to respond to those links as the default email application.

There used to be hacks that let a web browser with a webmail client open respond to mailto links but I haven't used one of them in years so not sure if/how that would work.