How to Fix Exchange Online Error 550 5.1.10 RecipientNotFound Bouncebacks
Question details
The user is receiving a bounceback error when attempting to send emails to certain external domain accounts.
- Product
- Exchange Online
- Device & OS
- not provided
- Scenario
- Sending emails to specific external accounts and immediately receiving a non-delivery report (NDR).
- Observed behavior
- Emails fail to deliver, generating a 550 5.1.10 RESOLVER.ADR.RecipientNotFound error, which indicates that the recipient address cannot be resolved by the receiving mail system.
Ensure you save a copy of the complete Non-Delivery Report (NDR) bounce message, as it contains essential diagnostic information needed by your email administrator or Microsoft Support.
Clear the Outlook AutoComplete Cache and Test on Web
This solution helps if your Outlook client has cached a corrupted or outdated email routing address.
Sometimes, the Outlook AutoComplete list stores broken background data for an email address. Forcing Outlook to look up the address fresh can quickly resolve local delivery failures.
Open Outlook and click on 'New Email' to open a blank message window.
Begin typing the recipient's name or email address in the 'To' field. When the suggested contact appears in the dropdown, click the 'X' icon next to it to delete it from the cache.
Type out the complete email address manually without relying on AutoComplete, and attempt to send your message.
If the issue persists, log into Outlook on the web (OWA) and try sending the email from your browser to determine if the issue is strictly related to your desktop client.
Remove Partially Created Domains in Microsoft 365
Use this solution if your organization previously set up the recipient's domain for testing or shared mailboxes but never completed the migration.
Verify DNS Records and Routing Configurations
Engage the recipient's IT administrator to check for incomplete DNS configurations or strict routing policies blocking your emails.
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Frequently Asked Questions
What does Exchange error code 550 5.1.10 mean?
Error code 550 5.1.10 (RESOLVER.ADR.RecipientNotFound) indicates that the receiving email server cannot find the recipient's email address. This typically happens if the address doesn't exist, has a typo, or if there is a routing configuration issue between the sending and receiving servers.
Can clearing the Outlook AutoComplete list fix this error?
Yes, clearing the AutoComplete entry for a specific contact forces Outlook to resolve the email address anew. If Outlook was trying to use a cached, outdated, or corrupted internal routing address, manually typing the full address will bypass the error.
Why would a partially created domain cause this bounceback?
If an external domain was previously added to your Microsoft 365 environment (e.g., for a planned migration or shared mailboxes) but never fully configured, Exchange Online will assume it hosts that domain. It will try to route the email internally instead of sending it over the internet, resulting in a 'Recipient Not Found' failure.
