I am Fastmail customer so I have access to their Sieve functionality but decided against it. For me the fundamental limitation is that Sieve looks at email only once, when email arrives. I wanted to do the things like: archive email after 3 days being in my inbox, or older than my most recent read email (which would mean I scanned the inbox and the summary was enough) etc. I ended up implementing a Python script that manages my inbox connecting to IMAP server on cron schedule. It also sends me telegram notifications from follow ups on starred threads.

Yeah, "keep sales newsletters for two weeks, then delete forever" has been a thing I've wanted for a long time. Nothing seems to make that easy.

I've been leaning towards local processing on an mbox folder... that would also let me build up multiple Bayesian filters to try categorizing my other mail - why does almost everyone only auto-categorize spam? Google kinda broke out of that with their "social" and "important" semi-categories, but it'd be pretty easy to build in local clients, yet I've never seen it.

For bayesian junk filtering to work, you train with messages marked either as junk or nonjunk. I suppose you could train based on positive tag, and assume the message has the opposite signal if it didn't get that tag. My historic email is only classified for (non)junk. If I would start more classifications, I would have to ignore the existing messages.

Fyi, I'm working on https://github.com/mjl-/mox, which also includes a bayesian filter. This sounds implementable.