Well, I can't reproduce your problem. So where do we go from here?

Do you position the cursor in any way other than by using the main keyboard? i.e. do you use return, space, letters and numbers? Or do you use the mouse or cursor keys to place it arbitrarily?

It uses html tags. If you get the cursor outside the current active tag, it'll go back to the "background" style.

How do you choose your font? Do you always try to apply it when composing? Have you tried setting your preferred font in Tools|Options|Composition|General?

Thanks for looking at it Zenos. There's not much more to say, I type emails the same way anyone else does, I can bold, underline, change size, ... do anything except change the font. If I click 'Reply' to something, the 'font box' initially shows 'DejaVu Sans Mono' (my chosen font), but with the first keystroke, it changes to sans-serif. Same if I start a new message. BTW, I have no 'Options' under 'Tools', this is icedove I'm using, I trust it wouldn't be different? So why no 'Options' then? I set up everything with 'Edit | Preferences'.

It seems that all I have to do is check the 'Allow messages to use other fonts' box! But why on Earth do I have to 'allow' *myself* to use the very font that I selected? When I select a font would one not expect that I would more or less automatically have permission to use it? I naively presumed that that box meant that I was permitting fonts *other than the one I myself selected*!!