the mail_daemon is called x-vnd.be-post in launch_rooster.
To end mail_daemon along with the Deskbar icon, you either stop it with launch_rooster if it is ran as a service or, especially preferable if you ran it on your own manually as an executable, you Shift + right click the Deskbar icon to open the extended context menu where "Shutdown mail services" shows up as the last.