[ from ftp://ftp.nikhef.nl/pub/network/README_FIRST ]

'vrfy' is a tool to verify email addresses and mailing lists.
In its simplest form it takes an address "user@domain", figures
out the MX hosts for "domain", and issues the SMTP command VRFY
at the primary MX host (optionally all), or at "domain" itself
if no MX hosts exist. Without "domain" it goes to "localhost".
More complex capabilities are: recursively expanding forward
files or mailing lists, and detecting mail forwarding loops.
Full-blown RFC822 address specifications are understood.
Syntax checking can be carried out either locally or remotely.
Various options are provided to exploit alternative protocol
suites if necessary, and to print many forms of verbose output.
Obvious limitations exist, but on average it works pretty well.
Needless to say you need internet (nameserver and SMTP) access.
See the man page and the extensive documentation in the source
for further details.
