You would think that email would work seamlessly between an Apple Mail user and a Microsoft Outlook user. Unfortunately, if you are just using the default settings on your Mac, you are likely to encounter Windows users who will complain that they cannot open your attachment or that you have "chinese" characters in your email text.
When sending rich text with attachments, Apple Mail produces messages with mixed encodings. When the message contains anything beyond basic ascii, like the accented characters used in European languages, it causes Windows Outlook to display these special characters as question marks, or other incorrect symbols.
To avoid this problem, you can send email as plain text instead of rich text. Alternatively, you can set Mail's default encoding to UTF-8. When running Mail, go to Message menu, select Text Encoding and select Unicode (utf-8). It is also a good idea to instruct Apple Mail to send file attachments as "Windows Friendly Attachments". When running Mail, go to Edit menu, select Attachments, and select "always send windows friendly attachments"
You can now freely email your Windows friends without having them complain about mis-formatted emails, or empty file attachments.