Push doesn't work

The push function does not work (the settings are all correct), nor does the manual collection of emails work.
The emails arrive at some point, but very often very delayed, sometimes minutes, sometimes hours.
What’s wrong with that?
Otherwise: I like the new design very much!

Without more details and maybe even logs it will be hard to help you. Device, Android and K-9 version are important infos, and logs never hurt.

Smartphone: Samsung A40
Android: 11
K-9 Version: 5.806
Logfile is not possible: ADB is not working, I get the answer:
“adb.exe: no devices/emulators found”
Although the smartphone is correctly connected, the device is not listed.
Tried several times.

Is your mail imap or pop?

It’s IMAP. I want to use push.

Have you confirmed that your mail hosting provider supports push, not all do. Who is your provider, someone in this forum may have experience with them.

Yes, my provider (GMX) offers push. I’ve already installed other email apps and tried to see if push works. It worked well, but other apps are missing some features that only K-9 offers. So I would like to continue working with K-9.