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Sender Verification

by jag Contributions Published on Feb 11, 2008 02:38 PM

As part of our spam control heuristics, we employ a commonly used technique called Sender Verification, which attempts to verify that an email message has a valid return path mailbox by testing whether an attempted mail delivery to that return path mailbox would succeed.

How does sender verification work?

A successful mail transaction on the internet using the standard mail transfer protocol SMTP might look something like the below. For ease of reading, I've prefixed lines sent by the client are by >> and lines returned by the remote mail server by <<:

<< 220 ESMTP
<< 250 Hello! What can I do for you today?
>> MAIL FROM: <>
<< 250 OK you are sending mail from
>> RCPT TO: <>
<< 250 OK you are sending mail to
<< 354 Enter message, ending with "." on a line by itself
>> To:
>> From:
>> Subject: Test messsage
>> Hello, Joe! I'm Bill Gates and I work for the FSF now!
>> .
<< 250 OK queued as 1JOehn-000219-Sy
<< 221, signing off

The numbers at the beginning of each line the remote mail server sends are status codes which indicate the result of each command. Code numbers in the 200's are messages that the previous command succeeded. Code numbers in the 400's indicate that the mail transaction failed due to a temporary error that may go away in the future. Code numbers in the 500's indicate a permanent failure has occurred and the transaction will not complete.

The simplest mail transactions begin with a HELO introduction of the server presenting the email, a sender return path in the MAIL FROM command, the email recipient in the RCPT TO command, and the email DATA itself.

According to the SMTP specification, the MAIL FROM command must either contain a null return path indicated by <> or a valid email address (enclosed in < and >) for a mailbox where delivery status messages can be sent. In the case of our example above, the return path was and any delivery status messages will be sent there.

But there is no such employee of the FSF named Bill Gates, and there is no such email address Such email should be regarded as spam.

Now let's say is upgraded to employ sender verification. It will probe to ensure that this address exists and accepts mail. So behind the scenes, after in our sample SMTP transaction the command RCPT TO is given, the server would contact the mail server for and perform the following SMTP transaction, referred to as a callout:

<< 220 ESMTP Exim 4.63 Mon, 11 Feb 2008 15:11:37 -0500
<< 250 Hello []
>> MAIL FROM: <>
<< 250 OK
>> RCPT TO: <>
<< 550 Unknown user

The server said 550 when asked if mail was deliverable to, and status codes in the 500's represent permanent errors. As far as is concerned, it now believes that does not exist, and it will reject the incoming message. The SMTP transaction would look like this:

<< 220 ESMTP
<< 250 Hello! What can I do for you today?
>> MAIL FROM: <>
<< 250 OK you are sending mail from
>> RCPT TO: <>
<< 551 Sender verification failed.

The FSF/GNU mail system uses sender verification.

Misconfigurations that Result in False Positives

Sometimes systems administrators have their systems misconfigured in a way that makes sender verification reject legitimate messages. We'll see how those work and how to fix them.

Misconfiguration #1: Automatic Emails from Nonexistent Mailboxes

Problem: Suzy Creamcheese ( registers for the Dairy Lovers' Blog hosted on and as part of that registration, she must respond to an email confirmation. The blog software generates the email containing the confirmation information automatically, and as a default it sends with the return path of, a nonexistent email address used for automated emails. When performs its sender verification callout, it is told that does not exist and rejects the message as spam.

The Misconfiguration: The administrator of the blog should not be sending these notices from a nonexistent mailbox.

What to Do: Write the administrator of the Dairy Lovers' Blog and inform them of their misconfiguration. If they wish to receive bounce messages from failed send attempts, they should provide a return path containing a valid mailbox. If they do not wish to receive such bounces, they should use the null return path instead.

Misconfiguration #2: Remote Mail Server Rejects Null Return Paths

Problem: Steven K. Eletor runs a mail server for his personal domain that he configured himself. In his misunderstanding of the internet standards, he configures his mail server to deny any emails that have a null return path. All sender verification callouts fail regardless of whether or not the mailbox they are inquiring about exists.

The Misconfiguration: Accepting mail from null senders is a violation of internet standards called RFC's.

What to Do: Write Steven K. Eletor and inform him that his configuration is not consistent with RFC 2821, the standard for SMTP transactions. He may be unaware that he has configured his server this way, or he may mistakenly believe that it helps control spam. Either way, in general, if he does not play by the rules of the internet as published in the RFC documents, he should not be surprised if other internet sites refuse his communications.

Misconfiguration #3: IP-based Blacklist Rejects Traffic Despite Null Return Path

Problem: Maria Fulano runs a mail server for an ISP that has vigilent spam control. It believes that mail originating from cannot be trusted to not be spam, so it sets up a policy to reject mail coming from that host. However it is so overzealous that it either denies the message with a permanent SMTP error before seeing the return path or despite a null return path.

The Misconfiguration: Null return paths are vital for sending delivery status notifications such as bounce notices. Even with IP-based blacklisting, mail with null return paths should be accepted.

What to Do: Contact Maria Fulano and let her know that she is entirely within her rights to reject mail traffic originating from IP addresses she believes to distribute spam, however it is unwise and unhelpful to deny such traffic prior to seeing the return path or despite a null return path. Contact the FSF/GNU sysadmins (or if your problem does not deal with an FSF/GNU mailbox, contact your mail administrator) and let them know about the problem so that they can also work on convincing Maria Fulano to un-blacklist the mailservers.

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