Self-Hosted Main

It kind of is a client that is connected, but at least one that you could run on a server without the need for a ui:

• https://github.com/lefcha/imapfilter

It being a console application means it does not come with a GUI, but rather a set of configuration files.

If you use Outlook, selfhost Exchange and you can set all the rules on the Exchange server.

WARNING: Selfhosting Exchange is a task you should not take lightly. You need to implement a few things to make it secure by default. Exchange had several issues in the past that allowed complete control of the Exchange server, which can be easily prevent if you host Exchange in the proper way! Besides security implications, Exchange is not a trivial software and requires a degree of knowledge to setup, but once you have it, you will never want to use anything else.

I have been after something similar to do both spam filtering and email organisation.

This one seems like it ticks the right boxes as well as the others commented

https://docker-mailserver.github.io/docker-mailserver/edge/config/advanced/mail-sieve/

The alternative if its just email filtering you could setup Thunderbird in a docker container so it runs 24/7 and you have a GUI to do your rules. So regardless of which other app you are using they get sorted assuming it gets them first.