Resolving SMTP Envelope Headers

In an effort to reduce the amount of spam hitting us, we've configured our mail relays to refuse to accept pieces of email whose SMTP envelope's From header isn't resolvable.

While this might prevent some folks from being able to get email in, we've taken the step because experience has shown that in the cases where the From header in the envelope doesn't resolve, this is the result of either intentional forgery or a misconfigured mailhost.

If you are having problems getting to us...

There is a solution to the problem. This will require that your return address resolve properly. Note that this isn't the From: header that you see in the email, but rather the From header that mail transfer agents (MTAs) use as the ``control address''. Please have your local system administrator fix your email such that your mail's envelope will contain fully qualified domain names (FQDNs) where applicable. (This is really something that should be done anyway.) We apologize for the inconvenience.

If you are a system administrator...

...and you're reading this because one of your users made you aware of this problem, but you don't know what we're talking about, here's a bit of an explanation.

What's most likely happened is that mail to someone here arrived at our gateway with a header in the form of user@host instead of a globally-resolvable user@host.domain. If this is the case, a bounce message would return to your user with an error message like:

    451 <user@mailhost>... Domain does not resolve - see for details and help.
This means that your mail relay is identifying itself as mailhost rather than mailhost.your.domain. How this is fixed will depend on your mail transfer agent and your own DNS. Given that this is very site-specific, we can't provide any more help about host configuration.

Certain mail user agents (MUAs) like Netscape, Eudora, etc., that want to use an ``SMTP host'' to deliver outgoing mail will write their own SMTP envelope. They'll do this by taking the value that the user provided as their email address (which will appear in the message's From: header and using it in the SMTP envelope From header. A mail transfer agent, configured to use the user's real email address, should be able to send mail to us without problem.

For more information, please see Why Spam Sucks for a more detailed discussion about why spam is bad for everyone.

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Matt Curtin
Last modified: Wed Feb 14 16:12:51 EST 2001