VRFY user@domain.tld
By default, VRFY will respond with a 252 response.
>>> VRFY user@domain.tld
252 2.0.0 Will attempt delivery
You can explicitly tell it to return a 250 response.
>>> VRFY pass=user@domain.tld
250 2.0.0 <[email protected]> OK
Or explicitly tell it to fail.
>>> VRFY fail=user@domain.tld
550 5.7.1 <[email protected]> unknown