I have been having the problem, too, for the last 5 weeks. Specifically, ever since I migrated from Entourage to the Apple "Mail" client.
A PC friend has been investigating the raw HTML that comes from my machine, and has decided that the extraneous "?" symbols (which replace any space-characters beyond a single-space character) derives from an "Apple converted space" command that appears in the HTML version, as in the following example:
[<]SPAN = class=3D"Apple-converted-space"[>]=A0 =A0 =A0 =A0
(note: remove the square-brackets from the above line. I had to insert them to get the complete command to display properly in this posting.) Here, the A0 is hex for the "?" symbol.
Somehow, Apple has to fix this. Like everyone else here, we're in a horrible fix when we send email to PC users -- we run the rist of looking quite unprofessional.