Unable to create a Spam Filter - Error 11006 pops up

Symptoms
When adding a new filter (incoming or outgoing) and filling in the mailserver IP the corresponding field - the filter is not created with a similar error message "('_ERR_SE_DESTINATION_INVALID', 11006); // Your request contains an invalid destination"
Cause 
The Spamexperts is not accepting the IP of the mailserver in this field. Same happens when we try to fill in IP in the Spamexperts panel.
Resolution
The mailserver hostname has to be filled in to the mailserver field (for instance, mail.example.com)