Version:           1.9.6 (using KDE KDE 3.5.6)
Installed from:    Mandriva RPMs
OS:                Linux

Kmail encoding configuration:
1-appearance page, message window tab:
     "show the state bar for HTML message" is checked
     "default encoding": utf-8, "overprint by": windows-1252
2-editor page, charset tab: list by priority order is: utf-8, windows 1252, iso-8859-1, iso-8859-14

Reproducing the problem:
1-create a new message with accented character (é,è,à...)
2-use html toolbar icons (U, B or S) to become this message HTML format
3-save this message as a draft
4-edit this draft message as if you wanted to complete it

Results:
é is became =C3=A9
è is became =C3=A8
à is became =C3=A0
â is became =C3=A2
ä is became =C3=A4