So I configured the quarantine... and it's working. I got some spam and banned message sent by some spam checker website and they didn't get delivered and appeared in the quarantine list.
I did come across a small bug though... I tried releasing a spam message, which did work, but right after that iRedAdmin Pro took me to
https://mail.domain.tld/iredadmin/activities/quarantined?msg=RELEASED <-- real domain name replaced for privacy reasons
showing only this error message on the page:
internal server error
It's not really a problem but still worth solving I suppose.
Cheers!