Fix Exchange 450 4.7.1 Unable to Relay ATTR37 Error
Question details
The user needs to resolve an email delivery failure where messages sent to Microsoft-hosted domains are rejected by the server.
- Product
- Microsoft Exchange Server 2016
- Device & OS
- not provided
- Scenario
- Sending emails directly to Microsoft-hosted recipient domains after changing the outbound connector or bypassing the Edge Transport server.
- Observed behavior
- The Exchange server returns a '450 4.7.1 Unable to Relay ATTR37' rejection message, indicating the outbound emails are blocked.
Ensure you have the exact public IP address of your sending Exchange server and administrative access to manage your organization's DNS and outbound connectors.
Delist the Sending Server's IP Address
Check if your Exchange server's public IP address is blacklisted by Microsoft's reputation service and submit a delisting request.
When an Exchange server's IP address is flagged for poor reputation or spam activity, Microsoft blocks it from relaying emails to its hosted domains. Using the official sender delisting portal is the primary way to restore mail flow.
Determine the public IP address of your Exchange server that is attempting to send the outbound emails.
Navigate to the Microsoft sender delisting portal at https://sender.office.com/ in your web browser.
Enter your email address and the blocked IP address, complete the CAPTCHA, and follow the instructions sent to your email to confirm the request.
Open a Microsoft 365 Support Request
If the IP is not listed on the portal or the error persists after delisting, request direct assistance from Microsoft Support.
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Frequently Asked Questions
What does the 450 4.7.1 Unable to Relay ATTR37 error mean?
This error indicates that the recipient's email server (hosted by Microsoft) has refused to relay the message because the sender's public IP address lacks a positive reputation or is currently listed on a spam blocklist.
How long does it take for Microsoft to process an IP delisting request?
Typically, Microsoft processes the automated delisting request within a few hours. However, it can occasionally take up to 24 hours for the updated IP reputation to fully propagate across all Microsoft data centers globally.
Can modifying my outbound connector trigger this relay error?
Yes. If you recently bypassed an Edge Transport server or modified your outbound connector, your emails might be routing through a new or different public IP address. If this new IP does not have an established sender reputation with Microsoft, it may trigger the ATTR37 relay block.
