After upgrading to Trados Studio 2021 CU5 or later, the following error message is displayed next to the email addresses already imported from Microsoft Outlook: "The specified email address does not exist". 

Go to File > Setup > Users and edit the invalid users with the correct email addresses. Alternatively, import the Outlook contacts again.

Trados Studio 2021 SR1 CU5 introduced an improvement in the way user emails are validated. This improvement is not compatible with Outlook data migrated from previous Trados Studio versions. You can re-import the Outlook contacts to address this problem.