Electronic mail involves a user agent at both ends (the sender and receiver) and two or more message transfer agents . We can divide a mail message into three parts: the envelope, the headers, and the body. We've seen how all three parts are exchanged using SMTP, the Internet standard. All three are exchanged as NVT ASCII characters .
We've also looked at newer extensions for all three parts: extended SMTP for the envelope, non-ASCII headers, and the addition of structure to the body using MIME. The structure and encoding used by MIME allow arbitrary binary data to be exchanged, using existing 7-bit SMTP MTAs.