People love to complain about Kmail, Akonadai, etc. but I just wanted to take a moment to say that I use Kontact every single day and I love it. I’ve tried the new Thunderbird, but I always come back to Kontact; I get my news from Akregator, Journal is a fantastic feature (and seriously overlooked part of the caldav spec) that I use to record meeting notes and ideas, Kmail is performant and set up exactly as I like it, and the Summary view is a great overview of what’s coming up. Sure, it could be prettier, but lord is it a performant, well-integrated option that’s absolutely bursting with utility. I’d encourage every KDE user to at least try to set up Kontact / Kmail etc. properly and give it a serious go. A development interest renaissance for KDE’s PIM stuff would be an absolute dream considering how strong the offering is today. Many thanks to those who put work in on this software!
What are the main benefits compared to Thunderbird?
The main benefit is that everything using Akonadi is that it’s kind of an all-in-one solution in KDE. The easiest example to show is that using the Kontact suite means you can also view calendar events in the time/calendar widget in your panel. If you enable the PIM plugin for that, of course 
Seconding @redstrate - it integrates super well with your desktop, minimize to tray by default, support for caldav journal, I also prefer Akregator to Thunderbird’s handling of RSS feeds, and I like the local anti-spam and anti-virus plugins in Kmail. It also just seems so much more flexible - Kmail has so many options that you can tweak.
Journal support is something I dearly hope comes to Merkuro as well, if it winds up being a Kontact replacement.
Does Kmail work with Gmail, in 2024?
I ask this since it did not work out of the box with gmail, when I tried it a few days ago. If I understand correctly, Google has blocked “less secure apps”, and Kmail is one of those. The app passwords thingy did not work as well, atleast when I tried. Whatever email protocols Kmail supports, I am pretty sure that supporting Gmail is a must, if Kmail is to become popular.
Also, does Aknonadi stress your system? When I used it a few years back, when gmail was supported, it sent the fans of my laptop spinning like crazy, for more than an hour or so. I couldn’t figure out what was the problem and had no other alternative than to completely reinstall the system. I want to give Kmail, or rather Merkuro Mail, a serious go again, but just want to make sure that I don’t get a similar traumatic experience again.
Fo Gmail integration, the email and calendar part have worked fine for me since I started using KMail a couple years ago. Contacts was broken but was fixed at some point last year.
Do you use an app password for kmail? Do you have 2 step verification turned on for gmail?
I use 2fa and the app password works fine
I haven’t personally had any of the issues you’ve described including Gmail, which seemed to work fine? But I’ve since dumped Gmail though and now run my own mail server.
Thanks. I gave Kmail a try again today, using 2fa and an app password and it works fine, atleast for now.