vrfy is a tool to verify electronic mail addresses using SMTP.

It recognises elementary syntax errors, but can do a lot more, up to complex tasks such as recursively expand mailing lists and detect mail forwarding loops.

In its simplest form, vrfy accepts an electronic mail address like "user@domain" for which it will figure out the MX hosts for "domain", set up an SMTP session with the primary MX host, and issue the SMTP VRFY command with the given mail address. The reply from the remote host to the VRFY command is printed.

If no MX hosts exist, it will try to contact "domain" itself. In case there is no "domain", the address is supposed to represent a local recipient which is verified at "mail", assuming the local system is configured to look up unqualified hostnames in some default domain that should support email.

By default only the primary MX host is contacted, assuming that "user" is local to that machine or that it otherwise has authoritative knowledge of all valid mailboxes.

With an option one may choose to also query all the other MX hosts.

For pseudo domains like "uucp" or "bitnet" one can compile in explicit servers to be contacted. They default to "localhost". Not many servers will tell what they are actually going to do with such addresses.

Instead of an electronic mail address one can specify the name of a file containing address lists, e.g. mailing list recipient files or .forward files. Verification of all recipients in the file is then attempted.

If an explicit additional host name is specified on the command line, verification is carried out at that host, and the input addresses are passed to the host without further parsing.

Various levels of verbose output can be selected. Very verbose mode prints the full SMTP protocol trace with the remote host. Even more verbose mode causes an additional SMTP VERB command to be issued, hopefully resulting in the display of all actions taken by the remote host when verifying the address. This can be fun.

In the special ping mode, the mail exchangers for the specified electronic mail domain will be contacted to check whether they do seem to respond to SMTP requests. No address verification is done.

vrfy has built in the basic address parsing rules of sendmail, so it can determine the domain part in complicated addresses such as: "comment "comment" comment" <user@domain (comment \comment)>

Elementary syntax errors are caught locally. If the domain part could not be parsed beyond doubt, the address is verified on the default "mail" host, hoping to get more detailed error messages from a real MTA.

Another option lets you recursively verify the received replies to the original verified address. This is handy for mailing list expansions, and also to detect possible mail forwarding loops. This works only by the grace of sendmail and other MTAs sending formal address specifications in the VRFY replies.

Recursion stops automatically if a local recipient address is received, or if a mail loop is detected. If the received reply is the same as the address that was asked for (modulo comments) the request is retried at its domain itself, unless this was the machine we just queried, or it is not an internet domain host.

The default recursion level is set to the MAXHOP value (17) as used by sendmail, but this can be overruled (smaller or larger).

Many SMTP servers do not support the SMTP VRFY and EXPN commands, or may reject those commands when they are issued by a "foreign" sender.

For this reason there is also an option to use the SMTP RCPT command instead of the VRFY command. This does not usually give the same information, but it is better than nothing. Usually the HELO or EHLO, and MAIL commands are required as well. Recursive mode is not possible.

Some hosts refuse to VRFY the bracketed "<comment user@domain>" but may accept the same address without the outermost brackets.

Usually hosts return addresses with an abundant amount of nested brackets if you present a bracketed address with comments.

An option will strip all comments from addresses to be verified, to avoid accumulating brackets during recursive verification. This is now the default when using the default recursion mode.

Some hosts return an error message, but with the 250 status code. As long as there is no '@' in the message, it can do no harm.

Some mailing lists have CNAME addresses, but we can now handle these, and do not get into infinite recursion.

Some hosts return an unqualified address for local recipients. This is acceptable if it consists only of the pure "local part" but sometimes it is of the form "user@host" which is difficult to trace further.

Eric Wassenaar was killed in a traffic accident on April 10th of 2000. He was the original author of vrfy, as well as of several other programs that were commonly known as the NIKHEF-NL network tools, after the National Institute for Nuclear Physics and High Energy Physics in Amsterdam where he worked and was responsible for their network operations and their connection to the Internet. His original web page at www.nikhef.nl/user/e07, and the FTP archive of his tools at ftp.nikhef.nl, was removed shortly after his death.

There is however a mirror of Eric's NIKHEF FTP tools directory available here: https://www.dfred.net/public/src/nikhef_nettools/

See the tribute to him in the NIKHEF 2000 Annual report.

I have maintained Eric's versions of host, ping', and vrfy` since then.