On-premises users can't get email messages from Microsoft 365 users in an Exchange hybrid deployment

Original KB number:   2730609


You have a Microsoft Exchange hybrid deployment in which cloud-based users in Microsoft 365 and on-premises users have the same email namespace (such as alias@contoso.com). However, on-premises users aren't getting email messages from Microsoft 365 users.

Additionally, when a Microsoft 365 user sends an email message to an on-premises user, the Microsoft 365 user receives a nondelivery report (NDR) error message that resembles the following:

#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##


This occurs if the domain that's set up in the hybrid deployment isn't set as an internal relay domain in Microsoft 365. To fix this issue, set up the domain as an internal relay domain:

  1. Sign in to the Exchange admin center as a Global admin or Exchange admin.
  2. Go to Mail flow > Accepted domains.
  3. Select the domain that's set up for the hybrid deployment, and then select Edit.
  4. In the Accepted Domain window, in the This accepted domain is section, select Internal relay as the domain type.
  5. When you're finished, select Save.

More information

The Exchange admin center lists the domains that you added to your account through the Microsoft 365 portal. It lets you manage how messages are delivered. In a hybrid scenario, Exchange Online must be set up correctly so that when a cloud-based user sends an email message to an on-premises user, Exchange Online routes the email message to the on-premises messaging environment.

For more information about accepted domains, see Accepted domains in Exchange Server

For more information about hybrid deployment, see Exchange Server Deployment Assistant

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