be sure to write your filters before receiving anything. Especially important if you're importing your email for the first time... and insanely annoying if you want to debug such an advanced filter.
as always with fastmail: It has great features... but always with a massive caveat.
One nice thing, for me at least, is it runs prior to the email ending up in your inbox and I am uses to Outlook rules were rules execute post arrival.
It would be awesome if some day there was a "jmapfilter". I think JMAP would be really efficient for this use case.
Also FastMail’s search works for any email header so it can be more potent than that of Gmail.
I wonder why can’t they add a checkbox to apply the rules retroactively. Seems to be simple enough.