This is not really an issue with Mac OS X but rather with the inability of Microsoft Outlook to treat text encodings properly. You are affected by this if you write emails in the mail application on Mac OS X and notice that replies to emails you have sent comes back with the replied text being all garbled. Apparently this stems from a couple of bugs in Microsoft Outlook that prevents it to treat the text encoding properly.
A simple fix to this issue is to set the mail applications to use Unicode encoding for outgoing emails. While this can be done manually for each and every email you send (Message -> Text Encoding -> Unicode (UTF-8)) it is better to change the mail application of Mac OS X to use Unicode as default encoding. To do this, open a Terminal window and type:
defaults write com.apple.mail NSPreferredMailCharset "UTF-8"