Answer
The VRFY clause is a method of verifying the existence of a user on a mail server. Normally you an either verify the existence of particular user or use a wildcard verify (VRFY *) to ask the server to return a complete list of users. The wildcard option can be exploited by spammers to bulk harvest email addresses so it is common to find administrators disabling this. The standard VRFY command (no wildcard) isn’t considered a security risk as indivdual e-mail addresses of course need to be parsed during an SMTP transmission.
Gordano’s products disable the VRFY (wildcard) command by default.
Keywords:RFC821, 821, SMTP, VRFY, protocol