Fix Exchange Hybrid Migration 451 STARTTLS Error
Question details
Users migrating to Microsoft 365 in a hybrid environment can send but not receive emails due to 451 4.4.0 and 451 5.7.3 STARTTLS errors.
- Product
- Microsoft Exchange Server
- Device & OS
- not provided
- Scenario
- Migrating mailboxes to Microsoft 365 in an Exchange Hybrid setup.
- Observed behavior
- Migrated Microsoft 365 mailboxes fail to receive messages, returning 451 4.4.0 or 451 5.7.3 delivery report errors stating that STARTTLS is required.
Ensure you have Exchange Administrator privileges for both your on-premises Exchange Server and your Microsoft 365 environment, and have your third-party SSL certificate details readily available.
Configure On-Premises Receive Connector and TLS Settings
The most common cause of the 451 STARTTLS error is an improperly configured receive connector or a missing TLS certificate on the on-premises Exchange server.
In an Exchange hybrid environment, secure mail flow relies on mutual TLS authentication between the on-premises Exchange server and Exchange Online. If the on-premises receive connector lacks the correct permissions or certificate bindings, inbound mail from Microsoft 365 will be rejected.
Log in to your on-premises Exchange Admin Center (EAC) using your administrator credentials.
Go to the 'Mail flow' section from the left navigation pane and select the 'Receive connectors' tab.
Select your Default Frontend receive connector and click the edit (pencil) icon to view its properties.
Navigate to the 'Security' tab and ensure that 'Transport Layer Security (TLS)' is checked.
Under Permission groups, ensure that 'Anonymous users' is checked so that Microsoft 365 can successfully route inbound emails through the connector.
Seek Assistance from Microsoft Support
If connector configurations are correct but the hybrid environment still fails to route mail, advanced troubleshooting via message traces is required.
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Frequently Asked Questions
What does the 451 5.7.3 STARTTLS required error mean?
This error indicates that the sending server requires a secure TLS connection to transmit the email, but the receiving server (usually the on-premises Exchange server) is not properly configured to accept or negotiate the TLS connection.
Why can migrated mailboxes send but not receive emails?
In a hybrid setup, outbound mail might bypass the on-premises server entirely depending on your routing choices. However, inbound mail from on-premises to Microsoft 365 relies on specific connectors. If the receive connector fails TLS authentication, inbound delivery stops.
Do I need a third-party certificate for Exchange hybrid mail flow?
Yes, Microsoft recommends using a valid third-party SSL certificate bound to the SMTP service for secure mail flow. Self-signed certificates are often rejected during hybrid mail flow TLS negotiation between your on-premises Exchange server and Exchange Online.
