554 5.4.12 Hop count exceeded in hybrid config

Matthew Taylor 0 Reputation points
2024-04-13T20:56:42.5033333+00:00

We have a hybrid environment with Exchange 2016 on-premises. After migrating a user to O365, we receive the NDR "554 5.4.12 Hop count exceeded - possible mail loop detected" when sending email from an on-premises user to the user we migrated to O365.

I have verified that the targetAddress property on the on-prem user is set to user@domain.mail.onmicrosoft.com, and that has replicated to the ExternalEmailAddress field on the cloud user. It would appear that the on-prem servers think they are authoritative for the mail.onmicrosoft.com domain, even though on the Accepted Domains tab on the on-prem server, I have specified that domain as Internal Relay.

Looking through the NDR shows that the message just bounces between my two mailbox servers, the CAS server, and the Edge Transport server. It never even attempts to send the message out to O365.

Any ideas?

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Microsoft Exchange Online Management
Microsoft Exchange Online: A Microsoft email and calendaring hosted service.Management: The act or process of organizing, handling, directing or controlling something.
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Microsoft Exchange Hybrid Management
Microsoft Exchange Hybrid Management
Microsoft Exchange: Microsoft messaging and collaboration software.Hybrid Management: Organizing, handling, directing or controlling hybrid deployments.
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  1. Bruce Jing-MSFT 610 Reputation points Microsoft Vendor
    2024-04-15T09:56:24.1866667+00:00

    Hi,

    I have a few suggestions for your description.

    1. Check the connector, in the EAC, navigate to "Mail Flow" -> "Connectors". Make sure you have a send connector specifically for sending email from Exchange on-premises to Office 365.
    2. Check that the MX record is pointing to Office 365.
    3. After running the HCW, update the Receive Connector on the Edge Transport server to ensure it will accept mail from EOP securely.

    We hope you can provide more information so that we can better solve your problem.