If you have VPOP3 retrieving messages from a POP3 account on an ISP, then you can use Download Rules to do basic filtering on incoming messages. (If you have incoming SMTP, then use SMTP Rules instead)
Download rules cannot perform all mail filtering tasks. For more features, use the Spam/Content Filter system instead, but download rules can do some things that the content filter cannot do, such as telling VPOP3 not to download a message at all, or to delete it from the ISP without downloading it first.
Download rules can only see the message headers, and other basic information such as the size of the message to be downloaded, and the current date/time etc. This is because download rules see the message before it has been downloaded (otherwise rule actions such as 'ignore' (don't download the message) would be meaningless).
To edit the download rules, go to Mail Connectors → Mail Collectors → (mail collector name) → POP3 General → Edit Download Rules. You must also have Use Download Rules checked for the download rules to take effect for this mail collector.
See the Reference section on Download Rules for more information.