Make plain vanilla the default option for all your Internet communications. I'd seen the rash of binary format messages on the list and contacted several people off line to try to debug it. I received copies of the offending messages in a number of different formats, depending on the make of the receiving person's machine. Each packet contained either a copy of the original message that the person replied to or a copy of the Internet mail routing data. The problem was manifested differently on each receiving machine types.
MIME types are standardized file formats that are used by specific software packages.
Since these are standardized formats, various software packages can import and export them and you can exchange files between similar software for various computer makers. Programs that don't understand the file format conventions see it as garbage. That way, we can use fancy fonts, underlines, graphics, etc and make it pretty. In an homogeneous office environment, that's usually true.