What type of failures are added to the suppression list?

Messages that have a failure response of 550, 553, or 554 and that do not match any of the signatures already in the suppressioncodes.list file will cause the recipient address to be added to the Suppression List.

The suppressioncodes.list file is located in the /bin directory. You can, of course, customize how certain responses are handled by creating a new suppressioncodes.list file in the /config directory.  Creating the file there will override the default configurations located in the /bin directory.