Enforced TLS between us and a client - SMG 8360 version 10.5.2.3

Under Protocols > Domains > I added the client domain > Deliver tab I selected Attempt TLS encryption. Now when i send them email i get this message

#< #5.7.1 smtp; 550 5.7.1 Unable to relay> #SMTP#

Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept e-mail from certain senders, or another restriction may be preventing delivery.

thanks