Messages can be stored on the system in something that is usually referred to as a "quarantine". Its most common use is for messages that are believed to be spam, which are both rejected (in order to notify the sender) and quarantined using a block in the DATA flow, or the HSL function Quarantine(). It can also be used for archiving.
System administrators can view and manage quarantines (folders) from the Activity > Mail tracking page (also available in a cluster) and choosing "Quarantine" as the Source. Messages in quarantine can be viewed, deleted or released (delivered).
End-users and domain administrators can access the system's (or cluster's) quarantine using the end-user web interface.