Cannot send mail from Exchange 2013 to Exchange 2007

I have anExchange 2007server with all roles in the same server (Version: 08.03.0327.001 ) on a Windows 2008 R2 server. I have now installed two Exchange 2013 servers both with CAS and MBX roles on Windows 2012 servers (full patched). There is also a DAG between the new servers. All mail goes through a BorderWare besides the internal mail.

I can send mail from both environments to the Internet. I can also send internal mail from Exchange 2007 to Exchange 2013.But the problem is that I am unable to send from Exchange 2013 to Exchange 2007. I've debugged, but not solved the problem and there are few articles on the Internet.

Any suggestions?