Spam, as you've already known, is any form of unsolicited or undesired email messages. An electronic form of junk mail. Taken from the food of the same name due to the common knowledge that people does not want spam (food) on a daily basis.
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Origins:
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Back in 1973 (you read that right). There was only Arpanet. Someone was working for a company engaged in computer technolgoes. He's in charge of marketing and is looking for a means of informing the people of his company's products. And in 1978, he thought of sending emails to around 400 people to market the product. And as was expected, a lot of PCs slowed down or crashed that day when users tried opening the unknown email with a long To:/Cc: list.
At the time, there are only around 1,000 users and data transfer is measured in baud. Average data transfer is less than 0.2 kbps over analog lines. And someone is sending emails to almost 80% of the...