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Sender Callback Verification

Why am I getting email bounces that say "451: Could not complete sender verification"?

It isn't us, it's the server at the other end. Before accepting email from outside servers, we verify that the sending address is reachable. We do this in order to:

  • Make sure mailservers talking to us conform to the internet RFC's (published standards).

  • Verify that if we need to send a bounce message back to the sender (as opposed to email getting blackholed with everyone wondering what happened to it).

  • Block spammers and viruses that send email with forged From: addresses (e.g. "")

  • We can disable this check for your domain, but two things will happen: Email for your domain will be slightly less reliable (since we can't verify whether bounces or replies will work), and you will start getting orders of magnitude more spam and viruses than you do currently.

    We recommend talking to the folks at the mailserver at the other end (the one trying to send the mail to you).



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