https://github.com/davepinch/pinchy.cc/blob/master/content/topics/films/aliens/nuclear-warfare/i-say-we-take-off-and-nuke-the-entire-site-from-orbit.md
Next, the CPYNET code was incorporated into SNDMSG. It remained to provide a way to distinguish local mail from network mail. I chose to append an at sign and the host name to the user’s (login) name. I am frequently asked why I chose the at sign, but the at sign just makes sense. The purpose of the at sign (in English) was to indicate a unit price (for example, 10 items @ $1.95). I used the at sign to indicate that the user was “at” some other host rather than being local.