The empty <plaintext> tag terminates the HTML entity. What follows is not SGML. In stead, an old HTTP convention specified that what followed was an ASCII (MIME "text/plain") body.
0001 This is line one of a ling listing
0002 file from <firstname.lastname@example.org> which is sent
The <plaintext> tag allows the rest of a file to be read efficiently without parsing. Its presence is an optimization. There is no closing tag. The rest of the data is not in SGML.