I was too late to edit, so commenting on self (sorry). The killer feature of Sieve, for me, is that the scripts are stored and run on the server; your mail is

I was too late to edit, so commenting on self (sorry). The killer feature of Sieve, for me, is that the scripts are stored and run on the server; your mail is sorted on delivery to the server. And because Sieve is a standard, you can port your scripts to a different server.