You need two connectors to route email between Microsoft 365 or Office 365 and your email servers, as follows: How do connectors route mail between Microsoft 365 or Office 365 and my own email server? Without connectors, email will not flow between Microsoft 365 or Office 365 and your organization's email servers.

If you have your own email servers and Microsoft 365 or Office 365, you must set up connectors in Microsoft 365 or Office 365. When email is sent between Bob and Sun, no connector is needed.(All internet email is delivered via Office 365.) When email is sent between John and Sun, connectors are needed.When email is sent between John and Bob, connectors are needed.John and Bob both exchange mail with Sun, a customer with an internet email account:

John has a mailbox on an email server that you manage, and Bob has a mailbox in Office 365. In this example, John and Bob are both employees at your company. The diagram below shows how connectors in Microsoft 365 or Office 365 (including Exchange Online or EOP) work with your own email servers. You can enable mail flow between Microsoft 365 or Office 365 and any SMTP-based email server, such as Exchange or a third-party email server. If you have EOP and your own email servers, or if some of your mailboxes are in Microsoft 365 or Office 365 and some are on your email servers, set up connectors to enable mail flow in both directions. How do connectors work with my on-premises email servers? To learn more about partner scenarios, see Set up connectors for secure mail flow with a partner organization. If you apply the steps described in this article to partner email services, you may have unintended consequences including email delivery failure. Before you get started, make sure to check on your specific scenario in I have my own email servers.

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