Many delayed messages since switching to Exchange 2013

We have been experiencing a lot of delays when switching to Exchange 2013. We currently use SpamHero as our spam filter. The flow is like this:

• Messages sent to Spam Hero
• Spam Hero sends to us which goes through our Cisco ASA 5540 firewalls
• Hits our CAS01 server which is just CAS and HUB roles
• Then goes to the mailbox server

We experience sometimes days worth of delayed messages and the delivery logs on our spam filter show this:

• status=deferred (conversation with outlook.domain.com[**********] timed out while sending message body)
• said: 451 4.7.0 Timeout waiting for client input (in reply to end of DATA command))

I seem to see it more with larger messages.