establish an SMTP sesssion and issue a "RCPT TO" or "VRFY" command and wait for a '200 OK' answer.

In our days, no sanely configured internet mail server will expose its account list by giving a clear positive or negative response to the VRFY or RCPT TO commands. Typically, the answer to VRFY is something like "I won't tell you" (my local Exim answers "252 Administrative prohibition"), and RCPT TO will be accepted no matter what mail account you try. The mailserver will decide after(!) the SMTP dialog how to handle that mail, and may send back a "mail not deliverable" mail. That may take hours or days, depending on the mail server configuration (greylisting).

Alexander