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.
exchange-2013-external-user-access-scenarios
how-to-configure-same-domain-zimbra-mail-amp-exchange-2016-mail
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