Who First Used the @ Sign in Domains and What Does It Mean?

Every day, we use computers and drive cars with no knowledge of what makes them tick.  Consider the humble “@” sign.  How many times did you use it today?

“Ray Tomlinson chose the @ symbol to tell which user was ‘at’ what computer. The @ goes in between the user’s login name and the name of his/her host computer.”  M. Bellis, “The History of Email” (ThoughtCo. Aug. 4, 2017); J. Freeman, The Tyranny of E-Mail:  The Four-Thousand Year Journey to Your Inbox (Scribner 2009), 2, 6; see generally, M. Berman, et al., eds., “Managing E-Discovery and ESI: From Pre-Litigation Through Trial” (ABA 2011), 752-60.