If you are receiving the error code 554 5.7.1 Sender address rejected: Access denied, This error is caused when a user is trying to send from our server without authenticating with a user name and password.
If you are using pop/imap you will need to go under Advanced Settings when setting up outlook and go under the outgoing server tab. It will have an option to "use same settings as my incoming mail server" this will need to be checked in order to resolve this error.