virtual domain can't recieve mail

[Amira Othman]

hi all;
I am using postfix-2.3.3-2.3.el5_6,dovecot-1.2.17-0_115.el5 on centos 5.6.I
have two domains and iam using virtual domains the problem is one domain is
working fin ewhile the other can't send mail when i check for mx record it
can't reach it.i don't know if i am missing some configuration or the
problem is with dns.any one can help me because iam new to linux and postfix
thanks