You’re inbox has a filter on it, that allows emails from friends anyone you’ve emailed to pass, but diverts emails that are just advertising or malicious to your spam folder.

Most of the time your spam folder will just contain junk, but sometimes mail that is important will accidentally get sent there.

So if you ever sign up for an account on a website, and the website tells you its sent a confirmation email to your account and you can’t find it, check your spam folder, it may be there.