Currently there is no way to track if an email invite sent to external users when they were added to a SharePoint site got bounced back due to incorrect email address or restrictions at the receivers end. We keep on waiting for the external user to accept the invite and they keep on waiting for the invite, but there us no communication in sharepoint or in O365 Message trace if the message was delivered or not. I even opened a support ticket and got the response that it is by design.