When the test user attempts to reply to an email which contains a contact record from a higher or cross Business Unit, there should be a warning or indication highlighting the specific contact. Ideally, the contact should either appear greyed out or display only the email address—similar to how it behaves in Outlook.

Suppose there are 10 contact records in the 'To' field of an email, and the user does not have access to one of them. In such cases, it does not allow the user to reply to the email. Users often find it difficult to identify which specific contact is causing the issue, and it takes considerable time to pinpoint the problematic record. Therefore, providing an indication—such as greying out the inaccessible contact while replying—would significantly enhance the user experience.