Two mechanisms: 1, the ability to write arbitrarily nested if all/any/none of various parameters (any/specific header w/restricted reflex) for server-side searches. The Mac mail app has these for the client side, as does my rss reader. A simpler grammar than needing inMailboxOtherThan yet more powerful. Could be represented as a json tree. 2 - the ability to run that filter n incoming mail basically where sieve is run today, with a similar small grammar for specifying a small set of commands to run like file in mailbox, tag, etc.