What determines what is spam in email, anti-virus or email?

For example, in Yahoo mail (which I have), does my anti-virus (which has anti-spam) determine what is spam or does yahoo?What determines what is spam in email, anti-virus or email?
Its yahoo mail who decide which mail is to send to spam or inbox.



"SpamGuard" is the name of Yahoo Mail's junk mail filtering system, which determines which emails to let reach your inbox, and which emails should be filtered and sent to your Spam folder.What determines what is spam in email, anti-virus or email?
the email client

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