I'm sending an email newsletter that contains Unicode characters - curly quotes, em dashes, and so on. When I send the newsletter to myself (from our custom content management system, to my account at Gmail) and then read the message in Apple Mail, the Unicode characters come through properly when Mail is set to Automatic in Message > Text Encoding, as they should given our Content-Type headers (for appropriate editions of the newsletter):
Content-Type: text/plain; charset="utf-8" Content-Type: text/html; charset="utf-8"
However, for a small number of people, including one who is also using Gmail (so the same receiving server), the Unicode characters are munged when Automatic is selected. If the message text encoding is changed to UTF-8, it works fine.
So what is it about Apple Mail, receiving the same message on the same server, that could result the automatic text encoding detection failing to pick up UTF-8 for the other person?