Sending emails from User on E2013 to a user on E2007.

I'm trying to send email from users that are on exchange 2013 to users on exchange 2007, i can send emails from E2007 users to E2013 users ok, 

I have followed all the previous answers but none seem to be working. 

im getting 451 4.4.0 Primary Target IP address responded with: "421 4.3.2 Service not Available........

Any help will be great, 

Thanks

William