The empty PLAINTEXT tag terminates the HTML entity. What follows is not SGML. In stead, there's an old HTTP convention that what follows is an ASCII (MIME "text/plain") body.
<PLAINTEXT> 0001 This is line one of a ling listing 0002 file from <any@host.inc.com> which is sentThis 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.