*this is quoted off of another site, and is not my work or saying:
"Note: If, trying to send an email, you receive the following error:
sorry, that domain isn't allowed to be relayed thru this MTA (#5.7.1),
it means that you cannot send an email before receiving (you need to start the POP3 service before SMTP).
I switched the "POP before SMTP" option in my email client on, and all started to work properly. If you find no such option in your email program (for example, Outlook Express), you just need to always press the "Receive" button before the "Send" button."
if you'd like to know the site pm me.
i just typed your error code and error in google and it popped up, but basically you need to start the pop3 service before you send a message through smtp. but all i did was type your error message "sorry, that domain isn't allowed to be relayed thru this MTA (#5.7.1)" in google and there are several links that may help...
edit - sorry, that was for a website hosting, but im sure it follows the same concept, i'll keep looking...