Hello again,
While looking more on the problem I found that courierd is not able to maintain the speed of queue parsing and feeding it's childs.
I've set into module.esmtp: MAXDELS=150 MAXHOST=30 MAXRCPT=100
And I still see that: # ps axf | grep -w courieresmtp | wc -l 48
Perfectly reasonable provided that most of your mail goes to a few major domains, like aol.com. MAXHOST=30 means that no more than 30 concurrent delivery attempts to the same domain.
It is also possible that because you're spewing virus-infested bounces, everyone else is tarpitting you.
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