Where does your mx records point to? What hostname, try to put that in there
http://technet.microsoft.com/en-us/library/aa996259(EXCHG.80).aspx
The value of the msExchSmtpFullyQualifiedDomainName attribute specifies the FQDN for the SMTP virtual server and must be resolvable by the Domain Name System (DNS) networking protocol.
A valid DNS HOST (A) record must exist for the value of this attribute so that messaging servers that try to resolve the FQDN of the SMTP virtual server can retrieve the correct IP address and route mail appropriately.
Sukh
Sukh