How to Customize the Exchange Online 550 5.1.10 NDR Message
Question details
An organization wants to direct senders to a new contact after an employee leaves, but senders receive an uncustomizable 550 5.1.10 NDR once the account is deactivated.
- Product
- Exchange Online
- Device & OS
- not provided
- Scenario
- Deactivating a former employee's mailbox and providing a custom redirect or contact message to external senders.
- Observed behavior
- Senders receive a generic Exchange Online 550 5.1.10 nondelivery report that cannot be customized because the recipient mailbox has already been completely removed.
Ensure you do not completely delete the former employee's mailbox yet. Once a mailbox is deleted from Exchange Online, the default 550 5.1.10 NDR cannot be customized.
Set Up a Temporary Mail Flow Rule
Create a temporary rule in the Exchange Admin Center to block incoming messages and reply with a custom explanation before the mailbox is fully removed.
By using a mail flow rule, you can reject emails addressed to the former employee and instantly send a customized response. This allows you to inform senders about the employee's departure and provide alternative contact information.
Log in to the Microsoft 365 admin center and navigate to the Exchange Admin Center (EAC).
Go to 'Mail flow' > 'Rules' and click on the 'Add a rule' button. Select 'Create a new rule'.
Under 'Apply this rule if', select 'The recipient is' and choose the former employee's email address.
Under 'Do the following', select 'Block the message' and then choose 'reject the message and include an explanation'. Type in your custom message (e.g., 'This person has left the company. Please contact [New Name] at [New Email]').
Save your changes and ensure the rule is enabled. It is recommended to keep this rule active for a transition period of around 30 days before permanently deleting the mailbox.
Use Automatic Replies (Out of Office)
Configure a standard Automatic Reply message on the account before it is deactivated to inform senders of the transition.
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Frequently Asked Questions
What does the Exchange Online 550 5.1.10 NDR mean?
The 550 5.1.10 Non-Delivery Report (NDR) is a standard error code indicating that the recipient's email address does not exist or could not be found in the organization's directory. This typically happens when a mailbox has been deleted.
Can I edit an NDR for an already deleted mailbox?
No. Once a mailbox is completely removed from Exchange Online, the system generates a default 550 5.1.10 NDR that cannot be customized. To send a custom message, you must implement rules before the mailbox is fully deleted.
How long should I keep mail flow redirect rules active for a former employee?
A transition period of 30 days is widely recommended. This gives frequent contacts enough time to receive the notification and update their records with the new point of contact before you permanently delete the mailbox.
