Exchange Server 2013 outbound mail flow problem 1 week after migration from 2007

I am in the midst of migrating from Exchange Server 2007 to Exchange Server 2013. After what I thought was careful planning, I was able to install Exchange Sever 2013 on a Windows Server 2012 VM. I migrated a small number of mailboxes over to the new server from 2007. The clients were able to send and receive mail for about a week when the outbound messages stopped and started piling up in the "Retry Queue". I am getting LED=441 4.4.1 Error encountered while communicating with primary target IP address:"Failed to connect. Winsock error code 10060, Wins32 error code 10060, etc (smarthost ip addr:25)" Attempted to fail over to alternate host, but that did not succeed. I have a 'Send Connector' setup to a smart host on both. 
Both servers are on the same subnet. 
I can telnet to the smarthost:25 from the 2007 server,
I cannot telnet to the smarthost:25 from the 2013 server. It times out.
I can telnet to and from each Exchange Server to the other on port 25.
Clients on the 2007 server can send and receive as usual.
Clients on the 2013 server can do all but send outside of the company.
The firewall on the 2013 server has been turned off.
This rules out the network firewall and ISP, because if port 25 was blocked it would affect both servers.
We did not make any additional changes, mail just stopped working after about a week.
NICs on both servers have identical settings for DNS, our two active directory servers.

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old286