Exchange 2013 to 2007 Mail Flow Not working

Dear All,

We are upgrading Exchange 2007 to Exchange 2013 Latest CU

All Pre-req are done, Co -Existence has been Established

All other functinality working fine , apart from mail flow from Exchange 2013 to Exchange 2007

Exchange 2007 to Exchange 2013 Works fine

I Tested Telnet from Exchange 2013 and Tried to send mail to Exchange 2007 and that also works , but when I am sending mail using outlook or OWA , mail gets stuck in Exchange 2013 queue and doesnt proceed 

Any idea what could be going wrong