How can the CISS Filter be used in a POP3 setting?
Enter pop3proxy.reddoxx in Configuration --> E-Mail Transport -> Local Domains.
In the "CISS" Tab, choose this domain and configure the CISS as usual.
Note that a CISS challenge only takes place when the client fetches the mail from the REDDOXX.
If the client is, for example, away on vacation, no challenge will be issued during this time.
Here, a central, server-based POP3 fetcher would be recommended.