Library for creating MIME messages
A tool that I have recently made use of is a library from Code Project. The library helped me to easily build MIME messages. As I was providing a high-level API to another caller I created a wrapper around the specific classes and methods exposed by this library.
The library can be found at https://www.codeproject.com/useritems/MIME_De_Encode_in_C_.asp. If you need to create MIME messages then check this out!