A very simple add-on for Thunderbird 68.2+ that uses native messaging to display a persistent icon in the KDE systray.
You definitely need a native binary in addition to the add-on. There is only one at the moment for KDE: a Qt5 application written in C++.
I suggest using the latest Thunderbird version 68.3.1, which fixes a bug in an API function when searching for unread messages. It may or may not work with earlier versions.
CPO is a tool written in Java that manages newsletters. It's focused on corporate needs (privacy, scalability, bounce count, opt-in & out, newsletter statistics).
IMAP-Agent is an automated IMAP mail folder management system implementing site wide and
individual user specific rules for filtering mail between folders.
Also, quotas can be maintained by age or max count of
messages for any folder.