Communication between Exchange 2003 and Exchange 2013

I want to allow communication between two internal exchange server, one is 2003 and the other in 2013.
I configured the DNS interior, with the mx records, to make sure that both servers can communicate.
From server 2013 to 2003 emails depart and arrive correctly.
From server 2003 to 2013 rather not start.
If I make a test with the telnet command from 2003 to 2013 the server everything works fine, but if I send the email through Outlook, the email remains in the queue and then fails to timeout.
If I do a nslookup from 2003 to 2013 comes out:
domaininexchange2013.net MX preference = 10, mail exchanger = x.x.x.x

(X.x.x.x is the IP of the mail server)

If I do a nslookup from 2013 to 2003 comes out:
domainInExchange2003 MX preference = 10, mail exchanger = x.x.x.x

(X.x.x.x is the IP of the mail server)

I do not understand why Exchange 2003 do not send the email if everything is correct.
Thank you very much, I hope you can help me because I'm frustrated

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oasis in my heart