SMTP used to have a command called VRFY which allowed you to verify whether or not an address was valid. RFC 2505 talks about how spammer abuse makes both VRFY and the EXPN command pretty much worthless nowadays: 2.11. SMTP VRFY and EXPN Both SMTP VRFY and EXPN provide means for a potential spammer to test whether the addresses on his list are valid (VRFY) and even get more addres…
