Mail remaining in outgoing mail folder

I use K9 mail on 2 Android devices, tablet and Smartphone. Both are in the same betwork using the same provider.
The tablet works well. Since some days mails I send from my Smartphone stay in the outgoing mail folder.

There are no incoming mails, too.
It happens that overnight the mails are sent suddenly. And, thre isn’t any errir message.

Of course, I compared the settings of both devices. They are identical. I deactivated my virus checker, too, but it won’t help.

I would appreciate any assistance.

Best regards

Check your phone’s system settings to make sure battery optimization for K-9 is disabled and background activity allowed (if you have that setting).

These are my settings.
And still, after a couple of hours, the mails are in the outgoing mail folder.
Would it make sense to uninstall and reinstall K-9?

K-9 Mail displays an error notification with some details when a message failed to send (make sure you’ve allowed K-9 Mail to post notifications). You can trigger another send attempt by going to the Outbox and selecting Send messages from the menu.

I have allowed K-9 to send notifications. Anyway, there isn’t any errror message and when I try to send a mail again from the outgoing mail folder that mail is added to this folder.

Now I have uninstalled and reinstalled K-9. A setup was found which certainly derives from my tablet (my problem happening on my smartphone ). I accepted and checked the entries comoaring with those on my tablet.
When I came to incoming mail setup and to check of the setup I had the following message 'setup failed. Details: failed to connect to (port 993) from (port 55972) after 30000ms".
I have no idea now what to do.

Do you really have a trailing / after When I try this to enter then K9 directly complains about invalid server name and are separate entities.

The app wasn’t able to establish a connection with the GMX mail server. There could be countless reasons for that. Are you using some kind of firewall app? A VPN app that blocks some traffic? Maybe your mobile provider blocks traffic to the mail server. Is the behavior the same on WiFi?

Isn’t it strange? When I just opened K-9 all mails having been stored in the outgoing mail folder have disappeared and have been sent. Furthermore I have reveived new mails. So it looks as if everything is allright in spite of the network connection error.
I’ll follow what happens and will address again if an error happens kater on.
Many thanks to all of you spending their time to help.