Hey guys,

do you know, whether there is any possibility to edit the source code of an email manually in KMail, before sending it? E.g. I would like to change/add the References and In-Reply-To field of the emails, I send, but I can't find how to do this.

Thanks in advance,
Cheers

You can't do that, but you can change headers (staticallly, unfortunately) in Configure KMail > Composer > Headers. For anything else you may want to look at filters.