Outlook can not send mail to external domains
I recently installed a RedHat Enterprise Linux system. I have sendmail, with cyrus being used for pop3 with outlook clients on my PCs.
The problem is that When I send emails from outlook to any external domains, I get the following error:
The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'firstname.lastname@example.org'. Subject 'Fw: Welcome to Outlook Express 6', Account: 'citygate.com', Server: 'citygate.com', Protocol: SMTP, Server Response: '550 5.7.1 <email@example.com>... Relaying denied', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79
To test my install, I used Telnet to port 25. Using this connection, I successfully created and sent an email to an external domain. I also used the regular Linux mail command to send emails to external domains without any issues.
It appears that the problem only occurs if I am using outlook and I am sending to an external domain.
At this point I am pretty much baffled. Any suggestions would be greatly appreciated.