Hi Andy,
Thanks for your reply and helpful suggestions.
Yes, I do (and did) have that set. The fact that this can be un-set completely does my head in. I assume that this ensure that you have a 3-letter Windows compatible suffix, among other Windowsy things.
Yes, plain text does solve the problem - but I don't want to send plain text, so this is not an acceptable work around either.
Basically, Apple Mail is very good, but needs a couple of fixes fairly urgently as far as I can see. And don't get me started on iCal and Outlook / Exchange compatibility!
Beren