Fix Microsoft 365 Emails Rejected in Exchange 2010 Hybrid Environment
Question details
Users need to resolve an issue where messages sent from Exchange Online to recipients in the same domain are rejected in a hybrid Exchange 2010 environment.
- Product
- Microsoft Exchange
- Device & OS
- not provided
- Scenario
- Routing internal domain emails from an online Microsoft 365 mailbox to on-premises mailboxes in an Exchange 2010 hybrid deployment.
- Observed behavior
- Emails fail to deliver to on-premises recipients in the same domain, generating a non-delivery report (NDR), often occurring when mail passes through a gateway like Barracuda.
Collect the Non-Delivery Report (NDR) to verify the exact error code and ensure you have Exchange Management Shell administrator access for both Exchange Online and the on-premises environment.
Verify Exchange Connectors and Approved IP Addresses
Check the configurations of both the Exchange Online outbound connector and the local Exchange 2010 receive connector to ensure proper mail routing and permitted IPs.
In an Exchange 2010 hybrid setup, the Hybrid Configuration Wizard creates specific connectors to facilitate mail flow. If these are misconfigured or if Microsoft 365 IPs are blocked, internal domain messages will be rejected.
Connect to Exchange Online PowerShell and run the 'Get-OutboundConnector' cmdlet to ensure the connector is actively routing mail to your on-premises smart host or Exchange server.
On your on-premises Exchange 2010 Hub Transport server, run 'Get-ReceiveConnector' in the Exchange Management Shell to confirm the Office 365 inbound connector exists and is enabled.
Ensure that the receive connector is listening on Port 25 and that the necessary certificates and permissions are assigned for anonymous or authenticated hybrid mail flow.
Verify that the latest approved Microsoft 365 IP address ranges are explicitly permitted on your Exchange 2010 receive connector network settings.
Configure Third-Party Mail Gateways
Update inbound connector and allow-list settings on gateways like Barracuda that sit in front of the Exchange server.
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Frequently Asked Questions
Why are emails from Exchange Online bouncing back with an NDR in my hybrid setup?
This commonly occurs when the on-premises Exchange 2010 receive connector or an intermediary gateway is blocking Microsoft 365 IP addresses, or if the inbound connector on the local server was not properly configured by the Hybrid Configuration Wizard.
How do I check if my Exchange 2010 receive connector is configured correctly?
You can verify the configuration by running the 'Get-ReceiveConnector' cmdlet in the Exchange Management Shell. Look for the Office 365 inbound connector and ensure it is enabled to accept connections on port 25.
Does the Hybrid Configuration Wizard (HCW) automatically fix receive connector issues?
The HCW is designed to create the necessary inbound receive connector on the local Exchange server automatically. However, manual verification is often required if you have strict firewall policies, a custom mail routing gateway, or if the wizard encounters partial deployment errors.
