When I receive new mails, i see a notification with K9 5.902. But if I delete this email on a desktop client, the notification is persistent and also visible after deleting the mail.
I don’t know what mail server software is used. The server is imap.strato.de
Notifications for emails deleted by other clients will only disappear once K-9 Mail learns that the message has been deleted. If you want this to happen in a timely fashion, enable Push. If you only use periodic polling, the notification will be displayed until the next time the folder is checked for new messages.
I understand. It seems that it took some minutes. And yes I have push enabled for this IMAP server.
It’s possible that the NAT timeout of your provider is lower than the IDLE connection refresh interval. In that case you might not be notified about changes on the server right away. Try lowering the value for Settings > [Account] > Fetching mail > Advanced > Refresh IDLE connection. However, you want to keep it as high as possible to avoid the device waking up more often than necessary.
I know, I can send via telnet some commands to the imap server on his port. Is there a command to find the right value or do identify what imap server software they use?
If not I will try to find it by trial and error…
The “right” value depends on the NAT gateway (of your provider), not the IMAP server.