If your Outlook Mail activity Report or Usage Reports are showing GUID strings rather than the actual email addresses in different sections of the report it may be due to privacy changes made by Microsoft when displaying 365 Usage Analytics. (see below).

This was introduced by Microsoft to preserve privacy in reports.

To solve this issue you should disable deidentification in the Microsoft 365 admin center → Settings-> org settings → Reports by UNCHECKING the box that says ‘display de-identified …’ . After that run must run the scan again.

This affects also SharePoint reports.

Reference: https://techcommunity.microsoft.com/t5/microsoft-365-blog/privacy-changes-to-microsoft-365-usage-analytics/ba-p/2694137