Hi abdy, thanks for your reply.
Yes, I'm onlny using IMAP protocol.
This night I also figured out something new about my problem. If a spam message arrives, i can find the following mail log entries:
spamd[15952]: spamd: processing message <embb9ef8d7-07b9-44f6-b5e9-88cf60b3f113@pc-joshua>...