Exchange 2013 not receiving internal and external emails ..

I have a coexistence of exchange 2007 and exchange 2013 ..2013 mailboxes where able to receive and send mails (internal and external) but suddenly the mail flow has stopped. 

Mail flow status

2013 to 2007 = OK

2013 to internet = OK

2013 to 2013 = OK

2007 to 2013 = FAIL

Internet to 2013 = FAIL 

incoming internet mails return the NDR below

What could be a possible reason for this? 

Cheers guys ..

..forever is just a minute away*