Do you have a "Contact Us" form on one of the websites you are hosting? There doesn't need to be an existing email account for a website to send emails, it could be sending the email using the PHP mail() function for example.
It sounds to me like there is a Contact form somewhere with no captcha protection, and it's getting submitted with junk contents, and the website was configured to email the forms to you.
The headers of the email may show you the originating PHP script that sent the email. Once you find it, you can disable or change it as needed.