Content rejected (TT992) means that TalkTalk has refused to accept delivery of mail from that mail sender. The bounceback will alert the email admins of the mail sending service that either the mail is not authenticated or the reputation of the mail sender requires improving to allow normal sending and delivery of mail.
Do you wish to say which mail sending service is involved? I assume mail is sent directly and not via any mail forwarding service when lack of authentication can result in rejection.
