@FarraigePlaisteach @aronkvh @ISOmorph
Most importantly what doesn't thunderbird have that kmail has? Akondai server.
Kmail worked so badly with larger imap mailboxes, that I switched back to thunderbird recently.
Thunderbird has its problems but it works. Only other linux mail client that worked similarly well is evolution/gnome mail but I did not want to have gnome services running on my KDE desktop.