Emails failing during migration from Zimbra to Exchange (same domain)

Miles M 1 Reputation point

We are migrating from Zimbra to Exchange 2019 in our domain. Exchange and Edge servers are already setup and seem to be able to send/receive emails between test accounts created within Exchange. We need to keep the DNS MX record pointed to Zimbra for now and slowly migrate users and their inboxes to Exchange. The problem is that in its current state emails going between addresses already in Exchange to Zimbra and vice-versa are all failing with the "mailer daemon undeliverable" automatic responses. I set up a receive connector pointing to Zimbra's IP already, my question would be is the problem because we have not switched the MX record to Exchange yet? Would that allow mail flow between the two but also have external emails go through Exchange and route to Zimbra if it's an account that has not migrated yet? Ideally I would like to see emails successfully sending and receiving between accounts on both sides before we flip the record but unsure if that's an Exchange or Zimbra issue.

Also...if I create a Send connector and point to Zimbra would that work and what specific settings should I use for that (intranet vs non-Exchange server)?


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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  1. LilyLi2-MSFT 1,981 Reputation points

    Hi @Miles M ,

    I am sorry to let you wait for such a long time.

    If you want to communicate with each other between Zimbra and Exchange. You could try to use Set-AcceptedDomain to set the domain type to an internal relay domain and create a Send connector for this domain to point to the Zimbra server. At the same time, you also need to create contacts for Zimbra users in Exchange and route messages using email a secondary domain.
    For split domain configurations with Exchange and Zimbra, please refer to this article.

    For more details, please refer to the following threads, and hope that helps.

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