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Dear Andrew J Cook,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, please don’t worry, we can work together to narrow down and resolve the situation.
Based on my test result, I also meet the same behavior as lie you described above. We cannot @tag to external users within Word documents to collaborators in comments in Word, Excel, when comment in a file and use the @-sign with external username. Even I tested in Word or Excel for web app but got the same result.
The default behavior is that @mentions does not work with guest or external accounts. Here is Microsoft Article for your reference. @mentions for comments in Word, Excel.
We feel your pain due to this limitation and we’re really feel sorry for it. Regarding to your concern, I request you, please send/provided feedback to related development team via Microsoft Word feedback portal, so that they can get notice about it, and we are sure it will speed up this process. Your advice is very important to us.
We can expect this feature to be used in the future in Word/Excel app
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Waqas Muhammad
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