We have two sites joined by a site-to-site VPN tunnel. Site A contains a Exchange 2013 server and site B a Exchange 2010 server. What we want to do is send mail from site A to a domain hosted on the Exchange server in site B through the VPN tunnel.
What we did is create a send connector for this domain on the Exchange 2013 (in site A) server and added the IP address of the Exchange 210 server (in site B) as the Smart Host.
On the Exchange 2010 server we created a receive connector which only listens to the internal IP of the Exchange 2013 server. Furthermore we set it to not use a Authentication Mechanism and set Anonymous users as the only Permission Group.
We are able to telnet port 25 from the Exchange server in Site A to the one in Site B, so that part seems to work.
When mails are send to a mail address in the specific domain (using OWA of Telnet) from site A, the mail gets stuck in the Exchange queue on the Exchange 2013 server.