relay not permitted, authentication required

As mentioned in the subject, that is the error when sending out an email. The domain is existed in /etc/virtual/domains already. With the same domain, another account can send out without any problem. Only one account cannot send out with that error message.

I also check in /etc/virtual/the-domain-name/passwd to make sure the troubled account there, and it's there.

I am using exim.conf 4.5.7

Any other place that I can check to find out the reason?

Thanks.