Email Errors: 451 / 501 / 553

Email Errors:
451 ... Sender domain must resolve.

This means that the domain part of your address did not resolve. The domain is the part after the @ symbol. This could be due to problems on the internet, so trying again may work. If it does not, contact your service provider for help. Your service provider is whoever you get your email, or internet service from.

501 ... Sender domain must exist.

This means that the domain part of your address does not exist. The domain is the part after the @ symbol. You may have mis-typed YOUR email address. If you cannot figure it out, contact your service provider for help. Your service provider is whoever you get your email, or internet service from.

553 Header Error,Message-Id Invalid.
You're either using broken mail software, or you're trying to spam us. If you're using broken mail software, you should get it fixed. It's putting out invalid email. If you're trying to spam us, go away.