Someone got access to my account and sent an email from it.
Never received a login from any other device, all devices were logged in has been checked (even the security camera) nobody got access through any of those devices. I got a reply to the email, so it is definitely my email address! How is this possible? HELP!
You must understand that anyone with sufficient knowledge can create and send emails with your ID. It does not indicate that the person has access to your account. What can happen is you enter a contest or sign-up on some site. They then sell your ID to a company that then rents or sells the ID to other companies or hackers.