I have been using KDE for a while, while I like many features I am looking for suggestions to the default email client:
Kmail - completely unusable for me and the only one which could maybe be integrated with kontacts, it could not receive mails from IMAP or pop or would receive only sometimes
Geary - good but too minimal, I need at least some kind of contact list and mailing lists feature, maybe this integrates with gnome contacts? I couldn’t find anything in settings
This recent post may be of interest to you: https://lemmy.ml/post/27474047
You may also find some ideas here or there.
I personaly use the power of neomutt and notmuch, but it’s not a GUI option if that’s what you’re looking for.