Mail gate show 452 4.3.1 insufficient system resources when sending to Exchange 2013 server

I just upgrade our Exchange 2007 server to 2013 and want to change the mail flow on my mail gate.

But after I change the mail flow to send email to Exchange 2013 server.

I got Failed [host xxx said: 452 4.3.1 Insufficient system resources (in reply to end of DATA command)] error on my mail gate sometimes.

I know this is about back pressure.

But my server have 16GB RAM and using 12.5GB.

Also the drive for mail queue have 100GB disk space and using about 70GB.

I cannot found any error about back pressure in the event viewer too.

I don't want to disable the back pressure check.

Please help me.