For example Rspamd spam filter. It works seamlessly with Dovecot. Dovecot can "detect" with sieve if user moves a message into junk or from junk folder into any other folder. And then passing this message to rspamd to learn from it if the message is SPAM or HAM.
If in Domino an event trigger is available to detect the user moves a messsage to/from junk folder then we can create the integration with third party spam filters to learn from this action by the user.