Software

I don't know how to solve it, but the message said that you are sending and e-mail to user@domain.com and you are sending the message to another server which is not domain.com. The server at domain.com thinks that the user lives in domain1.com (the other domain that you mention), but his is configured NOT to relay mail.
In other words, if using outlook express, i get connected to your smtp server (machine1@dom1.com) and try to deliver and email to juan@dom5.com, althogh machine1 may knows where to forward the message, it is configured not to do that.
WHY?? Because If I am a spam boy and you have a robust+high speed+good line server, I can connect (dial up to internet) to your server, deliver thousands of spam messages in a few seconds and then wait to see how your server is working for a week or so in order to deliver MY SPAM e-mails.

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