The first email message used the @ symbol to separate the user from the machine.
Why it is true
In 1971, Ray Tomlinson chose the @ symbol because it was rarely used in names and could indicate the user's location on a remote computer. This convention is still used in email addresses worldwide.
Source: Ray Tomlinson
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.