I would love to use the KApps for Calendar / Contacts / Mail for a better integration into KDE. But at this point Thunderbird is just so far ahead, I'm not sure KDE will be able to close the gap to make those apps a viable alternative
Thunderbird's entire GUI locks up when a notification pops up. KMail can't do that.
Thunderbird runs under Windows, KMail sometimes has at best an experimental Windows release but usually none at all. Decide for yourself how important Windows releases are for a Linux crowd.
I don't care for them but Thunderbird has extensions.
If they base Merkuro Mail on KMail (don't know if they do) I don't believe they lack much features. KMail was just a mess when it comes to settings and defaults, otherwise it's pretty great.
We are basing Merkuro Mail on kmail, and this is the current look of it.
People can test it by just running merkuro-mail from the CLI with 23.08 or even with 23.04 already by enabling it the config file. This is still WIP and some important feature like being able to send emails are still worked on. We need some helping hands so if people want to join our Matrix room is #merkuro:kde.org :)
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.