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
IMHO, you should consider doing more troubleshooting on Kmail. I’ve never used it personally, but from my understanding, it’s a stable program and shouldn’t have problems doing the basics of email, like you’re reporting.
The problem of kmail is the underlying subsystem that is evertything other than stable.