The at symbol was chosen for email because it meant at the price of.
Why it is true
Ray Tomlinson chose the symbol in nineteen seventy-one because it was used in accounting to indicate prices and was not used in computer system commands, avoiding conflicts.
Source: Email History
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Related facts
The '@' symbol is officially called the 'commercial at'.
The first email message used the @ symbol to separate the user from the machine.
The first email was sent in 1971 and used the at sign to separate the user name from the machine.
The first email ever sent was by Ray Tomlinson to himself in 1971, and it reportedly said 'QWERTYUIOP'.