Spam is unwanted mass-distributed electronic mail from strangers that arrives in your electronic mailbox, typically promoting products and services of dubious utility or legality.
Spammers have a variety of tools for gathering email addresses to build their mailing lists.
They may harvest your address from mailing list postings and web pages, sometimes with the help of software robots that retrieve email addresses and send them back to the host automatically. Just as with traditional junk mail, your address may appear on lists that are then sold by one company to another. To a certain extent it is the very openness and interactive nature of the web that exposes us all to the junk mail threat. A system with security so tight that you could not receive any junk mail would probably not be very useful as a communications tool. Fortunately there are ways that we can defend ourselves.