K-9 on Android, Thunderbird on desktop. I love both of them, but K-9 has some weird problem that background sync appear to be not working.

You mean Thunder? I think you are having some weird bug, because I’m logged there and have an account on lemmy.world. Jerboa recently was having problems with lemmy.world because of the server’s version, but that has already been sorted it out.

I’m currently using Thunder (it is open source, https://github.com/hjiangsu/thunder) and it’s pretty nice. I tried Jerboa before but I kept getting way too many request errors (probably because I was using with the lemmy.world instance tho)