Fix Microsoft 365 Email Forwarding Routing Loop Error 550 5.4.142
Question details
Users need to resolve a routing loop issue that prevents emails from being forwarded or redirected to another address within the same domain.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Configuring email redirection or forwarding to another address in the same domain.
- Observed behavior
- Email delivery fails and bounces back with a 550 5.4.142 RESOLVER.FWD.LoopingTarget error, indicating a circular routing loop.
Ensure you have Microsoft 365 administrator privileges, as modifying Exchange transport rules and running detailed message traces require elevated access.
Identify and Remove Circular Forwarding Rules
Check the forwarding configurations across mailboxes, contacts, and distribution groups to break the circular routing loop.
A routing loop occurs when an email is continuously forwarded back and forth between two or more addresses. To resolve the 550 5.4.142 error, you must identify where the circular reference is created and disable it.
Log in to the Microsoft 365 admin center with your administrator credentials and navigate to the Exchange Admin Center.
Go to 'Recipients' and select 'Mailboxes'. Open the affected mailbox, click on the 'Mailbox' tab, and select 'Manage email forwarding'. Ensure the forwarding address does not route back to the original sender.
Navigate to 'Recipients' > 'Groups'. Ensure that the forwarding address is not a member of a distribution group that also forwards emails back to the original mailbox.
Go to 'Mail flow' > 'Rules'. Carefully review active transport rules to verify that there are no automated redirection conditions creating an infinite loop between internal domain addresses.
Run a Message Trace for In-Depth Analysis
Use the message trace tool to see the exact hops the email took and pinpoint the exact source of the loop.
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Frequently Asked Questions
What does the 550 5.4.142 RESOLVER.FWD.LoopingTarget error mean?
This error occurs when Microsoft 365 detects an email being forwarded in an infinite loop between addresses. To prevent server overload, Exchange automatically blocks the message and sends a Non-Delivery Report (NDR) containing this error code.
Can inbox rules set up by end-users cause routing loops?
Yes. If an end-user sets up an Outlook Inbox rule to automatically forward emails to a colleague, and that colleague has a similar rule forwarding emails back to the first user, it will trigger a routing loop error. Both users' inbox rules need to be checked.
How can I analyze bounced message headers to find the loop?
You can copy the message header from the Non-Delivery Report email and paste it into the Microsoft Remote Connectivity Analyzer (Message Analyzer tab). It will break down the routing hops, helping you see the exact path the email took before the loop was detected.
