I had no idea this was possible. This sounds almost like an HTTP OPTION request. I'd love to find an example of client code which does this.

You're looking for the SMTP VRFY and EXPN commands. However implementation is very hit-or-miss. In the good ole' days of the internet, VRFY was widely implemented. Then spammers realized they could connect to a mailserver and do a form of a VRFY dictionary attack to find valid addresses, so it stopped being supported.