[Mimedefang] a case for supporting EXPN/VRFY on an internal server

[Gary Funck]

One of the difficulties we run into with MdF in general and greylisting
in particular is that recipient address verification (via the access
database) is delayed via delay_checks.  So, basically we tempfail messages
with invalid recipient addresses that we should reject outright
at the HELO phase, and these messages will have to be bounced later
if the sender retries.  Further, if we're front-ending other servers,
we may not even know whether the user address is valid.

But ... if the internal servers (including
a dummy server on the relay box for access_db checks) supported
VRFY, then MdF could check the addresses early on by first consulting
the internal server(s).

Make sense?