Be careful with whom you give your email address. Avoid sending personally identifiable information via email. Use two-factor authentication for extra security. Dont click on links in suspicious emails.

Why would a stranger want your email address?

It may give them information they need to steal money or uncover other personal information that can then be sold on the web. They can also mine your contacts list to send out phishing emails and/or malware to compromise even more accounts or defraud the people you know!

Is it bad if someone knows my email?

It is the most public piece of personal information you have besides your name. But what you may not know about your email address could hurt you. Many passwords reset via email, so even if you use a separate, long and strong password on, for instance, your bank account, a thief with access to your email can reset it.