I have reread the tutorials and everywhere else I can find to try and rectify this problem, even going so far as to reinstalling Postfix, but I cannot resolve:
"SASL authentication failed; server relay.dynu.com[168.235.105.136] said: 535 Authentication failed. Restarting authentication process."
I presume I should be using ar.......@gmail.com and my login password.
My sasl_passwd file contains:
[relay.dynu.com]:587 arkley101@gmail.com:dynu_login_password

The username and password is relay@relay.yourdomain and the relay SMTP password you set up for the service.