email: Multipart content type

RFC 2046 provides a specification for multipart content types.

1. Use Content-type: multipart/...; boundary=aBoundaryValue
2. Start each part with: --aBoundaryValue, that part's headers including the content type and a blank line.
3. End the final part with: --aBoundaryValue--

Notes

• The final <CR><LF> before each boundary line is considered part of the boundary line.
• For each message, choose a boundary identifier that does not appear in ANY of the parts.

Example

Content-type: multipart/text; boundary=AbCdE

--AbCdE
Content-type: text/plain

This is the first part (no trailing new line)
--AbCdE
Content-type: text/plain

This part has a single trailing new line
--AbCdE
Content-type: text/plain

This is the final part.  It has a distinguished boundary line.
--AbCdE--