Sieve is a powerful script language for server-side mail filtering. It is intended to be used with IMAP and thus it is widely spread. Many IMAP Server are capable of running sieve filters. Sieve stores and runs all script on the server-side. You can use telnet for this purpose, but that is far to uncomfortable, not applicable for a normal user and almost impossible with secure connections. Wouldn't it be great to activate, edit, delete and add sieve scripts with a convenient interface? That is exactly what the Sieve Extension offers...