Comment by wahern

RFC 1855, Netiquette Guidelines[1], specifies underscore for underlining. However, it says asterisks are for emphasis, not bold, per se. They just happened to (often?) display as bold because italics in terminals weren't a common thing. For the same reason, using /'s for italics didn't make much sense except maybe in word processors. I also suspect underscore become conflated with asterisk because some people preferred using the former for emphasis--people weren't usually trying to adhere to professional styling guides, and some people may have preferred underlining to impart emphasis, or just got into the habit without thinking about it.

I don't know how well RFC 1855 reflected common practice, though. It might be worthwhile to check the rendering code in clients like tin and mutt.

[1] https://datatracker.ietf.org/doc/html/rfc1855