Mails from certain domains not reaching virtual server


We have created accounts on a newly created virtual server(Redhat linux 3ES, OGL virtualmin 2.610). Mails into and from the new server seem to working alright with local servers and a few domains such as However mails from certain domains such as (which uses qmail) always bounce back with error as "User not known".
Surprisingly, mails from local servers are sent/received without the virtual server being bound to any "dns", and with no entries in /etc/hosts. Attempts to bind to a "dns" results in a total stop of the mail services. Kindly let us know if we are missing something vital, and help us.
Thanks in advance.