When I reply to emails then sometimes (but not always) the "in-reply-to" and "references" headers are missing. This leads to improper threading of mails in the receivers email clients. I found this in one of my email accounts that I manage in the same kmail instance. There it happens frequently when I reply to emails but not always. It is the same kmail instance in the same KDE session. I did not see a certain pattern yet why it is correct in some cases but missing in other cases. It seems to happen only in email threads with many recipients, though. The version is 4.14.6-0ubuntu1, this version was not selectable in the list of versions above, Reproducible: Always Steps to Reproduce: 1. Reply to an email, by default I use BCC to self 2. check message source if "in-reply-to" and "references" headers are there Actual Results: Sometimes the "in-reply-to" and "references" headers are there, sometimes not Expected Results: "in-reply-to" and "references" headers should always be there when the message was a reply to another message
It looks like there is a pattern after all. The issue only happens in smime encrypted emails and all such emails that I sent in reply to an incoming email are affected.
This bug has never been confirmed for a KDE PIM version that is based on KDE Frameworks (5.x). Those versions differ significantly from the old 4.x series. Therefore, I plan to close it in around two or three months. In the meantime, it is set to WAITINGFORINFO to give reporters the oportunity to check if it is still valid. As soon as someone confirms it for a recent version (at least 5.1, ideally even more recent), I'll gladly reopen it. Please understand that we lack the manpower to triage bugs reported for versions almost two years beyond their end of life.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.1 aka 15.12, preferably more recent), please open a new one unless it already exists. Thank you for all your input.