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I am using Azure with CentOS and Postfix and set up an account with a relay provider who banned me as a risk after 37 emails were relayed, but would not tell me why. I set up with another relay provider and incorrectly configured it, but the returned mail headers showed me the problem. A single email is being returned twice with different addresses, the correct address and one I have never seen before. This is as sent:

<[email protected]>: host smtp.elasticemail.com[54.38.226.140] said:
    530 Authentication required (in reply to MAIL FROM command)
Reporting-MTA: dns; RedberryCentOSServ.localdomain
X-Postfix-Queue-ID: 6AC6F2052DFE
X-Postfix-Sender: rfc822; [email protected]
Arrival-Date: Thu, 20 Oct 2022 16:55:09 +0000 (UTC)

Final-Recipient: rfc822; [email protected]
Original-Recipient: rfc822;[email protected]
Action: failed
Status: 5.0.0
Remote-MTA: dns; smtp.elasticemail.com
Diagnostic-Code: smtp; 530 Authentication required
From    Geoff Stow
To  Jasper Castillo
Date    Today 00:55
Hi Jasper. 

This is the rogue email:

<[email protected]>: host smtp.elasticemail.com[54.38.226.140] said: 530
    Authentication required (in reply to MAIL FROM command)
Reporting-MTA: dns; RedberryCentOSServ.localdomain
X-Postfix-Queue-ID: AE8B720C0F1F
X-Postfix-Sender: rfc822; [email protected]
Arrival-Date: Thu, 20 Oct 2022 16:55:09 +0000 (UTC)

Final-Recipient: rfc822; [email protected]
Original-Recipient: rfc822;[email protected]
Action: failed
Status: 5.0.0
Remote-MTA: dns; smtp.elasticemail.com
Diagnostic-Code: smtp; 530 Authentication required
From    Geoff Stow
To  Jasper Castillo
Date    Today 00:55
Hi Jasper.  

I have no idea how [email protected] is getting into the system given that I am using Rouncube as the email client and an Azure server. I can see nothing in postfix that could cause this also. This seems to be happening on all emails irrespective of the destination address. Any ideas anyone?

1 Answer 1

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530 Authentication required

These emails are being rejected because the account is not authenticated with the relay smtp.elasticemail.com, which requires it.

If this was you, you need to enable SMTP authentication. If it was not you, smtp.elasticemail.com is misconfigured and sending bounce messages over the public internet, a type of spam known as backscatter. In that latter case, it is fine to report the backscatter as spam.

If you don't recognize something, I'd guess there's an email forwarder involved and the forwarder is passing the bounce on to you (which I would still consider backscatter, but this time it's the forwarder's fault).

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  • The error 530 I understand, that is my fault and I will fix it. What I do not understand is why the same message to a single recipient bounced back twice, once with the correct recipient and once with a completely unknown address. This unknown address was also attached to emails sent to a different relay server.
    – Geoff
    Oct 20, 2022 at 18:43
  • My guess, as stated in my answer, is that the recipient forwards their mail and it bounced.
    – Adam Katz
    Oct 20, 2022 at 19:43

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