.he is just another word for .header in RUNOFF. Developed at MIT's Computation Center for the compatible time-sharing system in 1964, RUNOFF and its accompanying TYPESET editor were among the very first software to process text and its formatting. .he produces a headline, of course.
The Markdown markup language, some 45 years later, does not know .header for headers, but it knows to lay out headers, of course (#), and much formatting more you can use in emails with MailMate:
›› Add some visual emphasis to your words and text formatting to your emails. Here's how to use the Markdown markup language to send messages from MailMate that look good in both plain text and with HTML formatting.