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
Kmail, Thunderbird, Evolution. That’s pretty much it.
There’s always some weird niche client somewhere but it won’t be a hidden gem. Although I guess you can always use Pine (or rather Alpine nowadays) if you want to appear ubergeeky.