That's pretty much it. I have a newsletter that is sent to about 200 recipients in bcc. When I send this, it always gets stored in my local "sent mail" folder instead of the imap "sent" folder. All other mail get saved in the imap folders as expected and as set in "identities". Reproducible: Always
It seems there are bigger problems with messages with many bcc adresses, see https://bugs.kde.org/show_bug.cgi?id=334054
This bug has only been reported for versions before 4.14, which have been unsupported for at least two years now. Can anyone tell if this bug still present? If noone confirms this bug for a Framework-based version of kmail2 (version 5.0 or later, as part of KDE Applications 15.12 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.