Are the good, opensource email clients out there that check emails at regular intervals and configurable filters?
I've tried FairEMail and SimpleEmail, but they require opening the app to see emails. They seem to be forks or versions of each other without background syncing and (sorry) a terribly ugly interface. Neither seem to have filters.
Background syncing shouldn't be constantly on, just every 5-10 minutes or so. Normally I check once a day, but forget to quite easily and then miss an email. If it checked automatically even twice daily, that'd be enough.
Thunderbird?
On android? Couldn't find it on f-droid
https://f-droid.org/packages/net.thunderbird.android
it's basically a K-9 rebrand for now.
FairEmail pops up notifications of new emails for me
Your OS might be killing (outright or effectively) your app - check dontkillmyapp.com for possible solutions. Or the app doesn't have notifications enabled at the app and system level.
try K-9 Mail, it has a configurable background syncing: https://f-droid.org/packages/com.fsck.k9
Thanks. Will give it a shot!
I use FairEmail and I get notifications. I have it set to sync once an hour which works without issues. Maybe your android is killing it in the background? I also have a widget on my home screen showing recent emails, maybe that helps keep it alive?