I use Fastmail, and have found their Android app to be functional, but lackluster (no offline access being one of my biggest gripes). I hadn't used Thunderbird on the desktop in a good 15-20 years, so I figured I'd give this a try.
It is very snappy (unlike how I remember desktop Thunderbird all those years ago), and seems to work well so far, though it's a little bare-bones feature-wise. One thing that is unfortunate is it doesn't appear to be able to recognize Fastmail's "pinned" messages and map them to "starred" messages. That alone will keep Fastmail's own app on my phone.
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'll probably keep it and see if it can serve as my daily driver, with the Fastmail app as a fallback for pinned messages for now.
> 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.
It's not surprising that this feels different than Thunderbird as it's a completely separate app with a separate background just being rebranded with Thunderbird identity. There's probably some effort to make them play nice together, but K-9's been around for ages (I think I've been using it for more than a decade).
And very negative for the people that selected K9 precisely because of the peculiar UX. 5.600 has been the last good version for me. I still sideload it on every new Android device I buy.
It is very snappy (unlike how I remember desktop Thunderbird all those years ago), and seems to work well so far, though it's a little bare-bones feature-wise. One thing that is unfortunate is it doesn't appear to be able to recognize Fastmail's "pinned" messages and map them to "starred" messages. That alone will keep Fastmail's own app on my phone.
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'll probably keep it and see if it can serve as my daily driver, with the Fastmail app as a fallback for pinned messages for now.