> Fastmail also has different actions for marking as spam and reporting phishing; I'm not sure how effective the latter is, but of course Thunderbird doesn't support it, as presumably that's a Fastmail-proprietary thing.
I don't think there's anything proprietary going on, so I did a quick search. Fastmail supports JMAP [0], and JMAP [1] supports setting special keywords [2] for a message to mark it as spam of phishing. It may be possible to get this to work.
Ah interesting. Looks like that's strangely specific to JMAP and not a generic mail header or something like that, so K-9 won't be able to do it, as it doesn't (yet) support JMAP.