Is that implemented in the current rspamd version?
There is a request from colleagues to not reject extensions via multimap, but quarantine the mails in question.
You would need a WebUI where you can manage the quarantined Mails. At the moment thats not possible.
But what is possible, is that you can rewrite the subject of the Mails with a multimap alert. Mails with that subject can be redirected to another Mailbox.
But only possible via manual config changes - so you have to do it after every update.