One solution would be whitelist only. You must specifically whitelist a domain or email address, and everything else is blocked. If someone wants to send you

One solution would be whitelist only. You must specifically whitelist a domain or email address, and everything else is blocked. If someone wants to send you an email, you must get their email address. However, if you are using it to sign up for a service online, the domain often does not match the sender. You might sign up at the example.com website, but you get an email from noreply@auth-example.com as the email address.