advice on anti-spam tools

[Angelin Lalev]

Hi List,   
  
My e-mail server is running the latest spamassassin with all of the blacklist enabled and etc. 
but I still receive over 20 spam messages a day ("image" spam mostly). 
The situation with other users may be worse.  That's why I was thinking about some tool that 
1. store incoming email
2. send request to the sender of the message, requiring to go to some address and enter the numbers (letters) 
from image
3. if the puzzle is solved in time (week or so) deliver the message, otherwise delete it.

Is there such tool(s) ?