Mail Flow stops in Exchange 2010 after migration of Active Directory from 2003 to 2008 R2 64 bit

Hi,

we have migrated from 2003 Domain Controller to 2008 R2 and transferred all the roles new 2008 R2 Domain Controller we also have exchange 2010 CAS1/HUB1 & CAS2/HUB2 and Mailbox 1 and Mailbox 2 servers in the same LAN 

but after migration I removed the Network of old domain controller and I test the mail connectivity and mail flow internal email are fine but external going outside my organization stucks in the queue and doesn't go until I plugged back the old domain controller which currently ADC

Please someone help to resolve this issue I want all my mail flow to be fine with new migrated domain controller so that I can demote old DC

thanks 

Ehsan