There seems to be a problem where emails composed using RTF in Mail.app adds in characters that are unrecognized by Outlook (2000, in my test case). Outlook therefore just puts a question mark in those characters' places. To get rid?of the seemingly random?question marks, don't ever format your text "inline." Instead, do your formatting when you are finished composing the message.
In other words, don't use Command-I to enable italics before you type the word, and then hit Command-I again to turn off italics. Instead, just type your email all the way through to the end, and then go back and change what you want to change by hitting Command-whatever. For some reason, when you format your text while you're typing, Mail.app adds =A0 to the "end" of the emphasis in the RTF/HTML part. This can be seen from reading the raw info of the email -- I can verify this from within my Sent items in Mail by viewing the raw source! check this out:
I need a test email to see if I can get it to =
produce <i>weird</i>=A0characters,=A0within the email from apple's =
mail To ANOTHER client EMAIL ACCOUNT. <b>LET'S</b>=A0see if this does it =
ok.
All of those =A0 characters become question marks to Outlook users (at least, in Outlook 2000). This holds true even if you turn bold on and back off without typing anything, but then continue on composing, it seems. So, just wait till you're finished composing the email to emphasize your words -- and pray this is resolved in Leopard. BTW, I wound up looking into this due to this really old thread on the macosxhints forums.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070719071212177