Is there a way to implement a whitelist-based system in Gmail? Specifically:
- all messages coming from addresses in a "white list" get through;
- all messages coming that are responses in a pre-existing thread get through;
- all messages that include a special "passcode" string get through;
- the remaining messages trigger an automated canned response giving instructions on how to get added to the whitelist; these massages are automatically marked as spam.