Mail is showing me garbage for certain messages
It looks like your fonts got borked somehow.  This is most likely to happen if you've recently installed an application that adds or changes fonts, which includes apps like Office, iWork, the Adobe suite, and many more.

I'd start out troubleshooting Font Book.  Apple's KB article about Font Book has information about how to address this.

For troubleshooting, I think that you should try them in this order:
1. In Font Book, go to File > Restore Default Fonts. This is the easiest and seems the most likely to resolve your issue, since fonts in email are highly likely to be a default font.
2. In Font Book, validate your fonts (it's under the File menu).
3. In Font Book, check for duplicated font (it's under the edit menu).

Between each of these steps, quit Mail and restart it to see if it works; font changes won't show if you haven't restarted Mail.