Can't add domain!!!

[Dejin]

hi folks,
One of my reseller can't add domain;
He wants to add domain but he can't!
He added this domain as frame forwarding (he didn't write any disk space limit so it was 0 MB, 0 KB because it is frame forwarding but), then opened an mail account after that he couldn't see his domain on his domain list!
He tried to add again but he saw following error;

Error: Failed domain creation: Unable to update domain data: Failed mail post-configuration: Can't create mail domain servie: mailManager->addDomain() failed: mailmng failed: MEAOPO.PostOffice.AddPostOffice failed for name akgulcamayna.com [akgulcamayna.com].

Even when I am logging in as an admin, I have the same error!I can't add this domain I am having the same error!
Does anyone know what to do?How I can add this domain?

Thank you from now on!