Summary
The IMCEAEX error will arise when trying to send emails to a user who is no longer listed in the global address list of USNH. This article details the troubleshooting steps for addressing the IMCEAEX error within the Outlook Desktop App for Windows.
Environment
Microsoft Outlook Desktop App for Windows
Issue
You receive a "Delivery has failed to these recipients or groups" error in Outlook with "IMCEAEX" in the diagnostic information. This error is most often caused by a bad entry in the auto-complete list when trying to send a message.
Resolution
Clear Auto-Complete Entry for the recipient
If you receive this error, try clearing your auto-complete entry for the recipient you were sending to. (Auto-complete is a suggested email address that Outlook provides when you start typing a recipient's name or email address.)
Step 1 - Open a new message, and start typing the name or address of the recipient.
Step 2 - This deletes the cached address. Use To , to locate the correct address of the email if applicable.
Step 3 - When the auto-complete suggestion appears, click the X next to the name to remove the suggestion.
Step 4 - Another problem that could be causing this is that the person you are trying to send to has an email address that has been changed or been discontinued. You may need to reach out to them some other way to find out their correct email address. If they are part of USNH, you can try searching through the USNH Global Address List (GAL).
Step 5 - Type the recipient's correct email address or name into the To: field and try sending the message again.
Result
The IMCEAEX error should be resolved.
Further Readings
Outlook: Removing an Address from the Auto-Complete List
Outlook: Using the Global Address List (GAL)
Microsoft Learn - IMCEAEX non-delivery report when you send email messages to an internal user in Microsoft 365
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