Poco Junk Mail FAQ - Keeping only messages from known senders

Q: Any idea how to download all emails and then delete all those email except the sender's who is available in my address book?
 
A:
See the FAQ article on filtering by known or unknown email addresses.

Use the second example and either:

  • Move the messages to an "Unknown sender" mailbox (you will have to create the mailbox before creating the filter).
  • Change the filter action (perform) to "Delete".
I'd recommend the first method but for what you explicitly asked to do use the second.

Put this filter at the top of your incoming message filters. This will allow you to add additional filters later for messages from people who you do know.