I never sit still. I’m always coming up with something to keep myself busy, and I often end up complicating things. The upside is that I occasionally learn something new.
A few months ago, when I decided to switch from Mint to Debian with KDE, I also gave KMail a try. It’s an excellent email client, but it lacks an integrated calendar, so I had to use Merkuro (which isn’t quite ready yet) and later KOrganizer.
The problem was that neither KMail nor KOrganizer is available on Android, so I was using Thunderbird on my phone. That’s when I decided to use Thunderbird on my computer as well; I uninstalled KMail and moved on with Thunderbird.
However, for some reason I didn’t understand at the time, KMail kept sending me notifications about new emails even though it was no longer installed.
That’s when I learned that KMail relies on a host of other KDE services; even after the program is deleted, these services—which are shared by other packages—remain installed. One of them handles notifications.
The easiest way to stop receiving notifications from KMail, unfortunately, is to reinstall it and remove the email accounts and disable notifications in settings. There are threads in various forums going back a few years with users who have the same issue.
So, I had to reinstall KMail, remove my email account from it, and then uninstall the software again. This time, the notifications stopped, and now I’m only alerted by Thunderbird when a new message arrives.