Kmail 5.7.3 . Starting yesterday (July 20), any messages that I compose (receiving messages works properly)do not get sent. I have not made changes to my configuration.

Clues:
My OpenSUSE desktop runs on a wired connection. My wife's computer (same wired network) runs Windows 10 and she can send mail without a problem.I have a tablet that runs on wireless (obviously) through the same router and it can send messages properly. Using K9 mail on the tablet, e-mail sends perfectly. I have made no changes on the router, the mail settings on my PC or on the email configuration on my email provider's servers. (no password resets or changes, etc.)

I have run:
$ akonadictl fsck
$ akonadictl vacuum
$ akonadictl fsck

that does not help.

When kmail tries to send a queued message, there is a "Failed to transport message: server error" that shows up in the message preview pane.

If you have a solution, or if there is something that I should run to further diagnose the problem, please let me know.

Many thanks,
Mark