An email with 20 recipients. One wants to send an email to 5 of them. Doing "reply-to" on one contact works, creating a new composer window. In the past, it was then possible to drag other email-addresses from the existing email into the heading of the composer, drop them there and the email-address has automagically been added to the list of recipients in the new composer. This is VERY handy and unfortunately stopped working, probably with the move away from the KDE4 framework and QT update. This may be related to the Bug 368378 and Bug 36652 that have already been fixed by Laurent.
Hearing this is fixed in Opensuse Applications 17.08.4 or Kmail 5.6.3, this bug can be closed again. I will open another bug in the opensuse bug tracker