ERROR "relay access denied" in sending emails from a virtual mail user

[mahmudashraf]

I have installed ISPconfig 3 following The Perfect Server- OpenSUSE 11.1(ISPConfig 3] and things seems to work fine. It is a great work.
But, I have a problem with my virtual mail users.
I have defined a new mail domain and a new email box in that domain.
I can login to sqirrel mail with that email box and receive all my emails. But cannot send out any emails!
I get the following error:
-----------------------------------
Message not sent. Server replied:

Transaction failed
554 5.7.1 <user@some.mailserver>: Relay access denied
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Any one can give me a hint to solve my problem?

[till]

Is some.mailserver the local mailserver?

[mahmudashraf]

Thank you for you reply.
No, some.mailserver is a mailserver outside (e.g. gmail.com)
If I send an email to the local mailserver, there is no problem.

[till]

Please make sure that you have added the domain only as email domain in ispconfig and not as transport. Also make sure that you have not configured a relay domain in postfix main.cf.

[mahmudashraf]

Thank you for your quick reply. Please bear with me as I may have some dumb questions!
You said that:
"Please make sure that you have added the domain only as email domain in ispconfig and not as transport. "
Where can I add a transport domain? I even don't know how and I cannot find adding a transport domain in ispconfig.
Regarding the postfix main.cf file, I haven't touched it. My server installation is a clean out of the box installation.
I should add one note here that, I have asked our network provider to add the DNS entries including the MX ones in their DNS server. So, I guess I am not using mydns on local sever. Could it be related to my problem?

[till]

Where can I add a transport domain? I even don't know how and I cannot find adding a transport domain in ispconfig.

Email module > transports.

[mahmudashraf]

Thanks. I found it!
But, as I said, I didn't defined a transport domain.
Any new ideas about how to fix the error message I am getting?