In the olden days, when we trusted each other, (and had to send our mail uphill, in the snow, both ways), we had the VRFY command, and you could connect to an S

In the olden days, when we trusted each other, (and had to send our mail uphill, in the snow, both ways), we had the VRFY command, and you could connect to an SMTP server and ask it to verify a username. Most admins disable VRFY these days, and rightly so.