Many online accounts let you enter an email address that will be used if your password is ever forgotten or is changed. Problem is, some folks assume the email address will be used for spam, and so they enter a fake one. Or in other cases, the email address may have been valid at the time, but the account is no longer used.
It's important to know what email address you have associated with each of your online accounts. Make sure it is a valid and working email address and that the address belongs to an account that you routinely monitor.
Of course, phishing scams frequently masquerade as password reset email. Unless you initiated a password reset on your own, don't click any links or open any attachments to the email. Instead, login to the site the way you normally would and check the customer service or mail section for any notices about changes to your account. If you suspect fraud, report it immediately to the customer service department for that online account.