They can connect to a mail server and pretend that they are going to send a message and the server would reject the invalid recipient email.

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.

	▲	brk 4 days ago \| parent [-]
		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.