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 <pass=user@domain.tld> OK
Or explicitly tell it to fail.
>>> VRFY fail=user@domain.tld
550 5.7.1 <fail=user@domain.tld> unknown