This addresses the timing and double-download problems of client-side filters, but interaction with the filter is still awkward because it happens outside your e-mail program.In addition, you lose some control over the connections to the mail server and which messages are left on the server.
The e-mail program then “downloads” the good messages from the proxy.
Spam Sieve gives you back your inbox by bringing powerful Bayesian spam filtering to Mac e-mail programs.
It’s quick and easy to control Spam Sieve from within your mail program, and you can customize how it interacts with the rest of your message sorting rules.
After sending you a message, they get a reply asking them to complete a task that is easy for humans but hard for computers. This system is a nuisance for senders, delays your reception of the mail, and becomes impractical when sending messages to a group of people.
Also, challenge response systems cannot deal with spoofed senders or legitimate messages that are sent by programs.