It’s easy to ignore the postmaster group mailbox. It’s there to report on certain types of problems, which hopefully happen infrequently. However, if you don’t check it regularly and an unknown problem is causing it to fill will lots of messages (errors, bounce backs, etc.), then you can quickly find yourself running out of disk space.
An easy way to deal with this is to set an alert for the Postmaster group mailbox to email you whenever there is a message in the unnassigned folder that hasn’t been assigned. If you set this alert to trigger for just one message, then you’ll be notified everytime a message comes in, and every hour until you deal with it.
Once you’ve investigated the issue that caused the message, be sure to close or shred the message accordingly.