Version:           1.11.0 (using KDE 4.2.0)
OS:                Linux
Installed from:    Debian testing/unstable Packages

Currently there's no way to tell KMail if composition of plain text or HTML emails is prefered.

It seems like setting "Prefer HTML to plain text" in security settings is just about viewing, not about composition, although this is not clear from a user point of view.

It seems like changing a single email to HTML or back (e.g. by accedently pressing a shortcuts) makes KMail remember this and turn on/off HTML composition by default, but this is not reflected by any setting in settings dialog. I didn't even notice I was sending HTML emails until Jaime told me so in bug #180612... 

Thus the following proposition:

 - A "preferred mode" setting is missing for email composition. (IIRC this existed in previous versions, but I might be wrong).

 - When changing the formatting mode while compositing an email, a question could be issued with options "change default to xx", "only this email with xx, but stay with yy as default" and a checkbox "never ask me again". (Alternative: skip the warning and just leave the setting as it is).


Related bugs:
 - Bug #113778 - "Mess with HTML settings": Basically the same as this report, but less clear.
 - Bug #114683 - "Make KMail HTML-unaware permanently": Pretty much the same with emphasis on a missing warning.

Furthermore, bug #148915 - "Config file option to disable viewing (and crea..." goes even further and wants to disallow viewing and composition of HTML emails completely.