Claws mail is a very gui customizable email client, calendar, and RSS reader
It supports mbox/maildir, allowing you to use offlineimap.

Documentation

official docs https://www.claws-mail.org/documentation.php

homepage https://www.claws-mail.org/

Tutorials

vdirsyncer with clawsmail https://vdirsyncer.pimutils.org/en/stable/tutorials/claws-mail.html

official docs	https://www.claws-mail.org/documentation.php
homepage	https://www.claws-mail.org/