SMTP transports a mail object.  A mail object contains an envelope and content.
The SMTP envelope is sent as a series of SMTP protocol units.  It consists of an originator address (to which error reports should be directed); one or more recipient addresses; and optional protocol extension material.  Historically, variations on the recipient address specification command (RCPT TO) could be used to specify alternate delivery modes, such as immediate display; those variations have now been deprecated.
The SMTP content is sent in the SMTP DATA protocol unit and has two parts: the headers and the body. If the content conforms to other contemporary standards, the headers form a collection of field/value pairs structured as in the message format specification ; the body, if structured, is defined according to MIME.  The content
is textual in nature, expressed using the US-ASCII repertoire.
Although SMTP extensions (such as "8BITMIME") may relax this restriction for the content body, the content headers are always encoded using the US-ASCII repertoire.  A MIME extension defines an algorithm for representing header values outside the US-ASCII repertoire, while still encoding them using the US-ASCII repertoire.   _{Your Comment}