You should try AquaMail. The search works. Threads aren't shows as a tree, sadly, but they are grouped into a folder - not ideal but you can at least follow simple conversations. Still not something for mailing lists.
Additionally, AquaMail can handle nested folders, which K9 doesn't and never will, according to its developers.
It's not free software, unfortunately, so that is a showstopper for me. Thanks for suggestion, though.
Re:What Thunderbird Learned at FOSDEM
by PuffinBlue on Saturday February 10, 2018 @02:14PM
(#16348956)
Does it allow plain text composing/viewing yet?
Good question, this is also mandatory for me.
> search that actually works
If search works fast on Thunderbird is because it keeps all emails in your local hdd. To be fair you should also set the option account settings > fetching mail > local folder size to 'all' in K9.
That's a bit overkill for a phone.
I recommend enabling server side search in Account Settings, much easier on resources.
It doesn't work well either in my experience. Not sure if K9 is really the one to blame, or the IMAP server (Gmail, in my case). As Thunderbird indeed does search locally, it isn't impacted by poor server side search anyway. Thanks for the suggestion, though.
That being said, I'm rather happy with K9, and I'd certainly prefer to see energy spent in making it even better than in making a “Mobile Thunderbird” from scratch.