Am using Kmail 5.4.0
OpenSuse Tumbleweed

I discovered the issue after an update around Christmas time or shortly thereafter - I believe Kmail and other software was updated at that time.

Kmail and anokondi would not work - some errors about not being able to connect to the socket file. In the OpenSuse forum, I was advised to delete the socket file and try again.

That worked. At least it got Kmail running (not very well mind you and a major issue has been posted in another thread).

However, what I discovered was that Kmail was not deleting emails from the server. When I got kmail working, it was re-downloading thousands of email messages dating back to July of 2016.

In the settings I have:

Leave fetched messages on the server
Days to leave messages on the server: 7 Days

The latest version of kmail also is not deleting messages after 7 days so it appears to be a multi-version issue.

I am using SSL/TLS, Port 995 and Plain login.

Are you using POP or IMAP?

POP

Please file a bug on bugs.kde.org with the same information as here.