I have two Exchange servers - a 2010 server and a 2013 server. Both use my internal DNS for all name resolution. I can successfully send mail from 2013 to 2010, but not from 2010 to 2013. The error I am seeing in my queue for the version 15 hub transport is "451 4.7.0 Temporary server error. Please try again later. PRX2". In my experience, this points to a DNS issue of some sort. Both servers can ping each other. Port 25 is open between the servers. In my internal DNS, I have one MX record that points to my old 2010 server. Does communication between the servers depend on MX records? I've also made sure that the custom receive connectors on my 2013 box do not include the 2010 server's IP.