[stretch] KMail searching is bugged

[solitone <solitone@mail.com>]

A debian stretch user has recently reported a KMail bug on the kdepim-user 
mailing list. I also am on stretch I have the same issue, as well as a third 
stretch user. 

The KMail version in stretch is 5.2.3, installed from this package:

$ apt-cache policy kmail 
kmail:
  Installed: 4:16.04.3-3
  Candidate: 4:16.04.3-3

(BTW, how is the package version number related with the product version?)

Basically, the "Find Messages..." functionality you get when right-clicking in 
a folder does not work. However, if you delete the Last Search folder on the 
bottom of the folder list, it does work.

I want to send a bug report, but to whom? Do I report it to debian, or 
upstream to kde?

Thanks, 
  Davide