Thanks for the prompt reply. Your first suggestion unfortunately did not work. As for the second: Although I was able to get to the place in the Registry you referred me to, I don't know how to export or change the keys.
Perhaps it will help if I tell you what I see at the specific place in the Registry: There are 3 subheadings namely "Default", "New Theme" and "Template". Only the heading Default is surrounded by brackets and in the column "Data" the words "(value not set)" appear - also in brackets as indicated. As far as I can establish, the numerical values for ComposeFontComplex under (Default) are identical to the values for ReplyFontComplex under New Theme and TextFontComplex under Template. But in Outlook Options my chosen font sticks only for the reply font and the plain text font. Does this throw any light on the problem?
A further aspect: In the dialogue box in Outlook Options for setting the font for new messages, there is a block with the words "set as default" in the bottom left corner. However, this block is greyed out.
I hope the above will help to get to a solution.