Kmail contains a setting for the maximum number or recipients. If you paste more than this amount of recipients into the recipient field, it issues one error message per recipient. That may be some hundred messages to click away...one message in total would be sufficient
I confirm it.
Git commit 7605f3b754c523a6e473dbedc6fb3dc2745f09f2 by Laurent Montel. Committed on 10/09/2019 at 05:46. Pushed by mlaurent into branch 'Applications/19.08'. Fix Bug 411769 - Enter more recipients than permitted - error message for each recipient FIXED-IN: 5.12.2 M +8 -3 messagecomposer/src/recipient/recipientseditor.cpp https://commits.kde.org/messagelib/7605f3b754c523a6e473dbedc6fb3dc2745f09f2
(In reply to Laurent Montel from comment #2) > FIXED-IN: 5.12.2 That was a quick one - thank you Laurent!