Adds a new MIME part to multipart with the given headers and body.
(The multipart will make its own copies of headers and body, so you should free your copies if you are not using them for anything else.)
the MIME part headers
the MIME part body
Adds a new MIME part to multipart with the given headers and body.
(The multipart will make its own copies of headers and body, so you should free your copies if you are not using them for anything else.)