Tomlinson’s eyes fell on @, poised above “P” on his Model 33 teletype. “I was mostly looking for a symbol that wasn’t used much,” he told Smithsonian. “And there weren’t a lot of options—an exclamation point or a comma. I could have used an equal sign, but that wouldn’t have made much sense.” Tomlinson chose @—“probably saving it from going the way of the ‘cent’ sign on computer keyboards,” he says. Using his naming system, he sent himself an e-mail, which traveled from one teletype in his room, through Arpanet, and back to a different teletype in his room.

via The Accidental History of the @ Symbol | Science & Nature | Smithsonian Magazine.

The bang (!) was also used back then for email.