Spam mails still received although the CISS Filter is activated

Check if the sender is using a fake address to send mails to your domain or users belonging to your domain.
 
Activate the Anti-Spoofing Filter
 
1. Configuration -> E-Mail Transport -> Local Domains
Double click the domain and check the "Activate Anti-Spoofing" box.
2. Add the Anti-Spoofing Filter to the respective Filter Profile (move it to the top of the filter list for highest priority)
Make sure that the Anti-Spoofing Filter is NOT overridden by any Whitelists.
3. Remove your domain from the Domain Whitelist in the Administrator and User Whitelists. Do the same in the Address Whitelists.