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
I’m no Betterbird. For mail it’s just fine. OTOH, I find the calendar a bit annoying, as doesn’t allow to set email reminders when creating new event in a caldav calendary.