There are a lot of inventions seemingly invented in the wrong time – the tin opener, for instance, was invented almost 50 years after the tin. The first fax machine was created in 1843, and the @ symbol was made almost five centuries before the concept of tagging a celebrity you don't like and telling them that they suck.
Yes, the symbol most associated with email was first used, as far as we know, way back in 1536, with others arguing that it dates back even further to the 6th or 7th Century. Far from tagging Henry VIII in a very damning thread on the latest beheading, the symbol was either first used to mean "price per unit" or in pretty much the same way we use it now.
One theory is that in the 6th or 7th Century, the symbol was created by lazy monks. The idea goes that while copying endless manuscripts by hand, the monks looked for shortcuts. One shortcut was condensing the Latin word for "toward" or "at" (ad) into an "a" with the long back part of the "d" serving as the tail (@).
Another similar theory argues that French scribes first used it, swapping the French word for “at” (à), again to save time and effort. Rather than take their pen off the page, the scribes would lazily finish the "a", sweeping it up to form the "à", ending in the "@" symbol. In either scenario, the symbol means "at", just like today.
Another theory has evidence in the earliest documented use of the symbol. In a letter written by merchant Francesco Lapi in Florentine in 1536, the symbol was used as shorthand for "each at" relating to a trade (e.g. 14 lemons @ $10), mushing the a and the e together.
Eventually, this way of using the symbol fell out of use – but the @ button saw a revival thanks to computer scientist Ray Tomlinson in 1971.
Tomlinson was tasked with finding a way to send mail between computers connected to the Arpanet system, the precursor to the Internet. In solving the problem, he realized each person on the system would need an address comprising their name plus the name of the computer. In order to separate out the two parts of the address, he needed a symbol that wasn't used much in operating systems already.
“I was mostly looking for a symbol that wasn’t used much,” he told Smithsonian of the decision to choose the @ symbol. “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.”