More than 3.000 smtp sessions actives!

Hi,

I manage an only domain, with +-250 users. Sometimes, the MX handle up  
to 3300 smtp sessions

(netstat -an | grep ":25" | wc -l).

I have two MX, but only one work at productions same time.

I post recently a mail to this list about SYN_RECV connections. This  
problem only happen when I use my "qmail
based" MX.

In my qmail based MX, I've set a concurrency of 200, and I have the  
problem of SYN_RECV status. If I increase the concurrency limit, the  
system is too slow when I received thousand of spam.

But in my "kaspersky mail gateway based" MX, this problem doesn't  
happen (it can manage thousand of smtp connections without problem!,  
ex: 3300smtp sessions).

Have you any advice for me?

Thank you very much.