SUMMARY
Umlauts in a mail are broken when mail is send to a Kontact address list

STEPS TO REPRODUCE
1. create mail with umlauts in it (��� ��� ���)
2. select a recipient list from addressbook
3. send mail

OBSERVED RESULT
The text is completely destroyed:
Parkpl=E4tze sind vorhanden, die U-Bahn (U78/79) h=E4lt vor der T=FCre (Nor=
dpark/

EXPECTED RESULT
Parkpl���tze sind vorhanden, die U-Bahn (U78/79) h���lt vor der T���re (Nordpark/
Aquazoo)

Reason is that the message encoding in case of a recipient list is
Content-Type: text/plain; charset="iso-8859-1"
while when the mail is sent to an individual recipient it is:
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="iso-8859-1"

Needless to mention that this is really annoying and sheds a bad light onto
Free Software....

Operating System: openSUSE Tumbleweed 20210510
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.82.0
Qt Version: 5.15.2

Reported upstream: https://bugs.kde.org/show_bug.cgi?id=436987