IP count

Hi list,

I like to build a list of source IP's who send a lot of email to  
unknown users.
Does anyone know how to automate this, I can use something like:
cat /var/log/mail/info |grep "User unknown in virtual alias table"|cut  
-f3 -d [|cut -f1 -d ]|sort|uniq -d -c to get a list, but I want only  
entries where the IP  is listed for more than 50 times or so.

Any help is welcome.

Greetings,

Ariën