Zimbra utilizes a powerful spam filter to determine whether messages from specific addresses or domains should be added to your junk folder. Most filtering of spam e-mail is handled by the spam filter before those mail messages reach your Inbox.
E-mail that might be spam are filtered into your Junk folder. Therefore, sometimes a legitimate e-mail message can be put in the Junk folder when it is not spam. You should check your Junk folder periodically to ensure that you are not missing any messages. You can review these messages and either move them out of the Junk folder or delete them. If you do not move or delete the messages, they are purged after a number of days as specified by your administrator. You can empty the Junk folder by right-clicking the folder and choosing Empty Junk. In addition, the contents of the folder are automatically purged after an administration-configured number of days.
In addition to marking spam e-mails with the Spam button, headers from spam e-mails can be forwarded to firstname.lastname@example.org See Forwarding Email Headers
In addition to the automatic Zimbra spam filtering rules (above), you can setup your mail preferences to identify specific addresses to block or not block in your Inbox. You create a list of addresses that should not be added to your Inbox and addresses that always should be added to your Inbox, but could be detected as spam. The spam filter uses this list to make sure you get the e-mail you expect.
If you notice that certain e-mail addresses are always being incorrectly added to your Inbox or Junk folder, you can identify those addresses to block or not block.