Lot of SYN_RECV connections due to SPAM connections...

Hi,

I'm suffering daily attacks from an spam botnet. It isn't always at  
same time,  but occurs 3-4 times per day, during 10-20 minutes.

When it happen, my server hang up (at network level) with many  
SYN_RECV connections and main router is saturated because receives  
more connections that can handle.

Any fix?

I'm using this:

:allow,QMAILQUEUE="/var/qmail/bin/qmail-queue.kas3",RBL="",BLOCKRELAYPROBE="",RCPTCHECK="",BADRCPTDELAY="",SANITYCHECK="",SENDERCHECK="",SMTPAUTH="TLSREQUIRED"

Any protection "based on delay" the spammer it's self-defeating, since  
keep connections open during more time, will produce more connections  
was queued.

Thank you very much.