The action taken on the message (Allowed, Tagged, Blocked or Quarantined)
Tagged messages, with a keyword such as "[BULK]" prepended to the subject line, can be filtered by the subject line to a separate folder for later examination by the user (see the BASIC > Spam Checking page and the BLOCK/ACCEPT pages to configure spam scoring and criteria for tagging messages).
This error message is generated by Exchange and just relayed by POPcon. With this SMTP error message Exchange is telling us that no new emails are accepted because the harddisk space is less than 4GB.