Microsoft Exchange / Analysing backpressure issue

Hi,

We are migrating Exchange 2010 to Exchange 2013.
The mailflow still goes through the old Exchange 2010 and via internal relay to the new Exchange 2013 servers.
Yesterday i saw that almost every email that i can see in this queue gets delay with
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"452 4.3.1 Insufficient system resources"
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I immediately thought about "Back Pressure" and searched for entries in the EventLog.

After i couldn't find anything i disabled back pressure in the "edgetransport.exe.config" for some time, to proof that the issue is caused by that.
During the disabled state i had no delays or messages with that error  in the queue, so i'm pretty sure that the issue is caused by back pressure.
My problem now is that i don't have any idea how to find out which one of the back pressure thresholds is causing the issue.

Any ideas how i can deeper analyse this issue, to find out what the exact reason is?

Thanks in advance 
Simon