What is the meaning of: Max. Challenges per Sender?

This number determines how many CISS Challenges a sender becomes before the sender's mail is rejected without a valid response, unless the recipient delivers the mail him/herself.
 
The value prevents a spammer from receiving constant CISS Challenges and unnecessarily burdening the network and IT infrastructure.
 
The default value is 3. After the third mail is received from the same sender and a CISS Challenge has not been properly responded to, no more CISS Challenges will be generated.