and many MTAs do not accept messages from open mail relays.
The Internet email message format is now defined by RFC 5322, with encoding of non-ASCII data and multimedia content attachments being defined in RFC 2045 through RFC 2049, collectively called Multipurpose Internet Mail Extensions or MIME.
The body contains the message, as unstructured text, sometimes containing a signature block at the end.
The header is separated from the body by a blank line. Informally, each line of text in the header that begins with a printable character begins a separate field.
However, once the final restrictions on carrying commercial traffic over the Internet ended in 1995, Many MTAs used to accept messages for any recipient on the Internet and do their best to deliver them. This was very important in the early days of the Internet when network connections were unreliable.
Some early email systems required the author and the recipient to both be online at the same time, in common with instant messaging.
Today's email systems are based on a store-and-forward model.
Each message has exactly one header, which is structured into fields. The field name starts in the first character of the line and ends before the separator character ":".
The separator is then followed by the field value (the "body" of the field).