Kmail has its quirks, but I actually find it quite nice to use, especially with multiple accounts because it distinguish explicitly between IMAP accounts, SMTP accounts and identities.
Integration with contacts and calendars is quite smooth as well. KDE PIM suite is well integrated in Plasma in general.
I have been using for more than 10 years. Again, I’m not saying it’s without bug, but every time I tried to go back to Thunderbird, I quickly realise I miss Kmail’s interface and great integration in Plasma.