Viewing a user's activity
User does not have a connected mailbox or has never logged in
Look at the email details and email thread of that email activity. Check the recipients of that email thread. If there are other internal users in the same thread, then the email synced from one of their connected emails, and did not actually sync from the user that does not have a connected mailbox. The activity will show that user sending the email, but that activity synced from the other internal user's mailbox that is connected.