554 5.4.14 Hop count exceeded - possible mail loop ATTR34

Mountain Pond 1,411 Reputation points

Hello, I wanted to ask you how I should configure mail routing.

The organization is currently operating in a hybrid scenario. All messages are sent and delivered through Barracuda.
But we're finalizing the migration and want to switch all mail afterwards to o365 instead of On-Prem.
Now this is the scheme.

o365 currently has two connectors for the Barracuda. If we switch mail flow from Barracuda to o365. Users in On-Prem cannot receive messages with error: 554 5.4.14 Hop count exceeded - possible mail loop ATTR34
It turns out such a scheme.

Am I correct in understanding how to avoid this problem? I should create two other connectors instead of the incoming connector. So that all messages coming to domain.com (which is the accepted domain for the Exchange organization) are delivered to On-Prem. In this case, On-Prem receives or will be able to receive messages. And if the user is in O365, then his messages will be redirected back through on-prem to o365 using the technical address. To do this, a second connector is created that sends all messages to domain.onmicrosoft.com to o365.

Thank you.

Exchange Server Management
Exchange Server Management
Exchange Server: A family of Microsoft client/server messaging and collaboration software.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. Joy Zhang-MSFT 1,046 Reputation points

    Hi @Mountain Pond ,

    Based on your description, I want to confirm if you want your mail routing flow in below scenario: Internet mails - Barracuda -O365 -On-prem? Please correct me if I misunderstand your issue.

    As far as I know, if you want to internet mails flow to O365 first, you should disable centralized mail transport. Detailed information on routing incoming Internet messages through the Exchange Online organization for your reference.

    In addition, please refer to a blog on How to Configure Microsoft 365 for Inbound and Outbound Mail, and it indicates that you only need create one send connector to configure outbound mails. In the blog, it also mentions that each of your domains from which you want to be able to send email must be added to the Barracuda Email Security Gateway. Be sure to add all of your accepted Microsoft 365 domains to the Barracuda Email Security Gateway before configuring outgoing email in this section.
    Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.

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