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I am working on an odd issue that is occurring over the past few days with receiving undeliverable email errors. The error is 550 5.7.367 ("<email> is not authorized to relay messages through the server that reported this server").

The odd part is, this error is actually being shown when emails are sent from different domains (a Microsoft365 tenant and, here's a blast to the past, a @msn.com domain). Different devices are being used, as well (laptop/phone).

My only thought is that, where these emails are sent from, the ISP recently changed from Comcast to Hotwire Communications who laid out new fiber lines. My thought is that there is something on the receivers end of the email that doesn't like the new IP address or ISP, but personally have never ran into this issue before and figured someone here may be able to guide me in the right direction! I'm considering setting up a VPN connection to mask the IP, but I am unsure whether this will actually do anything.

I should note that the emails are being sent both from and to different domains and email addresses with the same result.

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Generally, "Relay Access Denied" error happens when the mail server is unable to authenticate the mail user, meaning that the user's outgoing mail server (SMTP) fails to correctly authenticate its emails.

The most likely reason is a DNS issue such as incorrect MX record or incorrect SPF record set up by the new ISP, which caused the sent email to bounce back with this error.

You need to ensure that the new ISP has a correct DNS setup, possibly by contacting their Support team.

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  • Thank you! So I was thinking some sort of DNS issue, but, personal Microsoft/Outlook Online accounts and Microsoft365 would have the wrong SPF/MX record setup? Seems a little odd, although I would 100% agree if it was an on-prem server
    – malypr
    Commented Jan 19 at 21:14
  • If that user is relaying their email via a server of the ISP, the problem is with this server, or with the authentication that this server reports for their email. In this case, the problem needs to be solved by the ISP, not by the user.
    – harrymc
    Commented Jan 20 at 8:56
  • The emails originate from Microsoft cloud services. The ISP does not host relays where this email would be routed through. Unless you mean like DNS lookups of remote mail servers are failing on the ISP level? I'll get the DNS record changes and see if that helps.
    – malypr
    Commented Jan 20 at 23:12

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