Espresso

Espresso permissions-based email client completely eliminates spam as you define it from your Inbox.
Espresso's powerful email filter is based on a sophisticated challenge-response system.

All messages from approved and trusted senders are immediately accepted. Espresso doesn't rely on deep header analysis, anti-spoofing techniques, heuristics, or Bayesian filters to eliminate spam. It simply looks for the messages you want to receive using sensible, yet powerful rules. With a selection of options available, you decide how to handle messages from unknown senders. Espresso delivers all the email you want to read and blocks all the email you don't want. It's that simple!
In addition to managing spam, Espresso's full-featured email client offers:
- lightweight and portable application
- powerful junk mail filtering based on permissions
- filter messages using regular expressions
- ability to add notes to individual messages
- hide private messages from prying eyes
- automatically blind copy on all outgoing mail
- easily flag important messages
- supports POP3, IMAP4, HTTP protocols
- preconfigured for easy setup of Hotmail and GMail
- built-in backup and restore mailbox
- powerful spell-checker for outgoing messages
- choose to save or delete messages when you send them
- automatically mark messages as read when moved to trash