You usually get this error (550: relaying denied) if you want to send an e-mail through our system without having authenticated yourself at the SMTP server before. To authenticate at the email server, you either have to retrieve new messages from the mailbox just before sending the emails or you use SMTP-Auth (also called ASMTP) to log in at the SMTP server.
Most e-mail programs configure the mailbox automatically, but this does not work with all programs. With Microsoft Outlook, for example, server requires authentication must be activated.
IMPORTANT NOTE: If you still get relaying denied, please check first if the correct authentication type is set. Correct is Plain or Login. The method Cram-MD5 is NOT supported!