[aasasd]

There's an issue on their tracker that discusses implementing the notification, and docs that say that without it K9 can only check mail every 15 minutes or so, even if excluded from battery optimization. So I don't know what support for IDLE and ‘background ability’ you're talking about. Before Android 9 or so, sure—it could just run in the background and connect to the servers whenever it wanted.

[muppetman]

Interesting - I can't say I've encountered this but I'm going to check the bug tracker now. Thank you - I have updated my parent comment.

[aasasd]

Here it is: https://github.com/k9mail/k-9/issues/857

The issue is even older than I thought, though I was pretty sure the requirement for a notification specifically appeared around Android 8 or 9.

Also, seeing as K9 doesn't look to be anywhere near frequently updated, I'm not expecting this to change soon.