'vrfy' is a tool to verify e-mail 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, * detecting mail forwarding loops, * understanding full-blown RFC822 address specifications, * syntax checking that can be carried out either locally or remotely, * various options provided to exploit alternative protocol suites if necessary, and to print many forms of verbose output.

Obvious limitations exist (mostly due to a lousy VRFY command implementation on some hosts), but otherwise, it works pretty well.