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Home » Kerio User Forums » Kerio Connect » Result: delayed, Status: 4.1.1 451 Could not complete sender verify callout

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Hello everyone,

We have been seeing a few of these messages in the mail log of our Kerio 6.2.2 mail server -

Result: delayed, Status: 4.1.1 451 Could not complete sender verify callout

When trying to send email to a few (not all) different domains. Is there something I should be checking on our end? It seems like our server isn't set right but I can figure out what it is.

Thanks for any help!

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I saw a number of messages similar to this the other day, except in my case, the messages had been rejected.

A company we deal with just implemented an anti-spam system which does call backs to verify the address the mail is coming from. The IP address they used for the new system was blacklisted. The blacklisting caused KMS to reject their verification attempts, which caused the verification system to reject our delivery attempts.

Systems that perform call backs are evil things. It's got to be one of the most problematic and least effective anti-spam systems I've ever seen.

Call backs work in the following way:

  1. You connect to a remote server to deliver mail.
  2. The remote system takes the address you are delivering mail from, and connects back pretending to send a message to that same address.
  3. If they get an address OK response to the TO address (the FROM address from your message), they accept your mail. Otherwise, they reject it outright, or temporarily fail it.

A delayed message with a response of '451 Could not complete sender verify callout' could mean they weren't able to successfully connect to your system to perform the verification. They may have connected, but didn't wait long enough to get a response.

You could enable the SMTPS and SMTPC debug logs. You could then see if they connect back to you while your system is trying to deliver mail to them. Make sure the mail is being sent with a legitimate e-mail address.

Other than that, I'm not sure there is a way to correct an issue like that, other than to scrap the system on the remote end that's using it.

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