exchange 2010 2013 smtp relay

Have Exchange 2010 topology in place currently with:
(2) CAS servers (using CAS array and windows NLB)
(2) Mailbox servers (using a DAG)
Barracuda Spam firewall used for transport to/from outside world.

Just spun up
(2) Exchange 2013 servers (with both cas and mbx roles)
using Barracuda load balancer for cas balanced access
still using Barracuda Spam firewall for transport to/from outside world.

My issue is relaying using the new Exchange 2013 servers
We have several other services who relay through our existing Exchange 2010 cas array address and send email both to internal and external addresses correctly.

When I change the other services to relay SMTP through the new Exchange 2013 barracuda load balanced cas I can send emails to mailboxes that reside on the Exchange 2013 servers, but NOT to mailboxes that reside on the Exchange 2010 mailbox servers, nor to the outside world.

I can send from mailboxes on 2010 servers to mailboxes on 2013 servers.
I can relay through 2010 servers to the outside world and internally to both 2010 and 2013 mailboxes
I cannot send from mailboxes on the 2013 servers to mailboxes on the 2010 servers.
I cannot relay through the 2013 servers to the 2010 mailboxes (only 2013 mailboxes) nor the outside world.

I'm not ready to move all the mailboxes over yet from 2010 to 2013 (due to voicemail integration).

All other CAS functions work using new Exchange 2013 servers (OWA, ECP, etc).

It's as if the emails relaying through Exchange 2013 to mailboxes on Exchange 2010 just disappear, I cannot find them in mail flow traces.

There is a relay connector on the 2010 cas servers which allow certain network subnets to relay. The 2013 exchange servers are in those allowed subnets.  

I have not made any changes to the initial receive connectors.
jasonbrandt3Asked:
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jasonbrandt3Author Commented:
I tried adding a relay connector on the exchange 2013 servers as per this article:
http://www.petenetlive.com/KB/Article/0000891.htm

via a telnet session all looks good, mail is queued, but never gets delivered.

Emails from a 2013 mbx user to a 2010 mbx user get Delivery is delayed message:
"Remote Server returned '400 4.4.7 Message delayed'
 11/13/2014 7:48:37 PM - Remote Server returned '451 4.4.0 Primary target IP address responded with: "451 5.7.3 Cannot achieve Exchange Server authentication." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
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jasonbrandt3Author Commented:
I found my answer here:
http://jermsmit.com/primary-target-ip-address-responded-with-451-5-7-3-cannot-achieve-exchange-server-authentication/

had to allow exchange server authentication on the 2010 connector relay I had in place
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