As far as I know, it is not currently possible to use @mentions with guest users in Microsoft 365 files. However, there are workarounds available. One option is to use the guest user's email address in place of the @mention. Another option is to use the comments feature in Word or Teams to communicate with the guest user. To do this, select the text you want to comment on, then click the "New Comment" button in the "Review" tab. You can then type your comment and the guest user will receive a notification.
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