Hi Nico,
kSpam works with the email before it gets to the mailbox, while it's still "in memory". Most AVs work with the message by intercepting the message before it gets to the mailbox and marking it as dead, then the AV addin scans the email while it's in the mailbox before removing the dead flag.
The only part which would conflict between kSpam and your AV would be when they both want to have access to the email while it's still "in memory", just before the message is saved to the mailbox.
I'm sure that Domino solves this conflict by giving access to the first extension manager addin listed in the ExtMgr_Addin notes.ini parameter, then the second addin listed, then the third ….. etc.
You may solve problems you are having by re-ordering the addins listed on the ExtMgr_Addins line in your notes.ini file.
-tom