Fix Outlook 550 5.1.11 IMCEAEX Exchange Bounceback Error
Question details
Users receive a 550 5.1.11 IMCEAEX non-delivery report (NDR) bounceback error when trying to send emails to internal users.
- Product
- Microsoft Outlook
- Device & OS
- not provided
- Scenario
- Sending an email to an internal user after a mailbox migration, name change, or hybrid Exchange environment update.
- Observed behavior
- Email delivery fails with a 550 5.1.11 IMCEAEX non-delivery report (NDR) because of a bad LegacyExchangeDN reference in the Outlook auto-complete cache.
Before troubleshooting, verify if the bounceback issue occurs with a single specific recipient or multiple users, and obtain the complete non-delivery report (NDR) for reference.
Clear the Recipient from the Outlook Auto-Complete Cache
Use this straightforward solution if you are an end user experiencing the bounceback error when emailing a specific contact.
Launch Microsoft Outlook and click on 'New Email' to open a blank message window.
In the 'To' field, begin typing the name or email address of the recipient who caused the bounceback error.
When the recipient's name appears in the drop-down auto-complete list, click the 'X' icon next to their name or press the Delete key to remove the cached entry.
Click the 'To' button and manually search for and select the recipient from the Global Address List (GAL) to ensure you are using their updated routing information.
Create an X500 Proxy Address (Administrators)
Exchange administrators can resolve this globally by adding the old LegacyExchangeDN as an X500 proxy address for the migrated user.
Resolve RESOLVER.ADR.ExRecipNotFound Issues
If the non-delivery report specifically mentions 'RESOLVER.ADR.ExRecipNotFound', you may need to force Outlook to update the Offline Address Book (OAB).
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Frequently Asked Questions
What causes the 550 5.1.11 IMCEAEX error in Outlook?
This error is typically caused by an outdated LegacyExchangeDN reference stored in the Outlook auto-complete cache. When a mailbox is migrated or renamed, its internal routing address changes. If Outlook uses the old cached address, Exchange cannot find the recipient, resulting in a bounceback NDR.
Can I fix the IMCEAEX error without an IT administrator?
Yes, if you are an end user, you can usually fix this issue locally by deleting the specific contact from your auto-complete list when typing a new email, and then re-selecting them from the Global Address List.
How do I convert an IMCEAEX address to an X500 proxy address?
To convert the address, you must extract the IMCEAEX string from the NDR. Replace all underscores (_) with slashes (/), replace '+20' with spaces, and add 'X500:' at the beginning. An Exchange administrator can then add this resulting string to the user's proxy addresses to resolve the issue for everyone.
