It looks like bug #285945, when I try to send an email, I got this error : "There were problems trying to queue the message for sending: Append failed". I don't know how I can find more information, I tried to launch kmail using konsole, but I got no errors inside it.
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.