vrfyallow, vrfydefault, vrfyhide)
These keywords control the PMDF SMTP server's response when a sending
SMTP client issues an SMTP VRFY command. The vrfyallow
keyword tells PMDF to issue a detailed, informative response. The
vrfydefault tells PMDF to provide a detailed, informative
response, unless the channel option HIDE_VERIFY=1 has been specified,
as discussed in Section 23.1.2.2 . The vrfyhide keyword
tells PMDF to issue only a vague, ambiguous response. Thus these
keywords allow per-channel control of VRFY responses, as opposed to the
HIDE_VERIFY option which normally applies to all incoming TCP/IP
channels handled via the same SMTP server.