How does Gmail keep out spam?

I heared that Gmail keeps spam out of your inbox. How does it do it, what does it keep out, and does it work?

How does Gmail keep out spam?
It looks at several things, what domain it came from, who sent it, if the header has been tampered with, phrases and words in the text, images being downloaded. It also figures out that if something like Gmail gets an email or multiple emails that are identical and it is being sent to hundreds or thousands of recipients to be spam. Also it tends to self learn as time goes on on what are likely to be spam.

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