Exchange server 2013 - updating to SP1 then CU5 result to mail flow issue

Hi,

I'm facing a huge issue with our exchange server since we applied SP1, mail flow is now stopped and our front side server (postfix with spam assassin) get error : 451 4.7.0 Temporary server error. Please try again later PRX4.

Regarding same post about this issue over forums, it looks like Transport service isn't working pretty well but that's not our case as all MS Exchange services are started.

The only thing is the HubTransport Component which is in "draining" status and I can get it back to Active using command :

Set-ServerComponentState -Identity mailserver -Component HubTransport -State Active -Requester Maintenance