Since some time it seems like you can't open new mails when they arrive by clicking on their notification. I think formerly there was a button which opened the mail directly (see Bug 374917). (plasmashell 5.12.3)
There's a setting for that: Settings → Configure KMail → Plugins → Email notification → Show button for opening email
Oops thanks. I'd suggest to enable this by default. Or is there any disadvantage in showing these buttons?
I wish one could just click the notification itself but the maintainer didn't like it so that's how it is now.
Yes, that would be like notifications of other applications are working (e.g. Firefox).
Is there any chance that this will be changed? Otherwise you can close this issue.
*** This bug has been marked as a duplicate of bug 412579 ***