Legal definition of email


Is there anything that defines email for legal purposes? Some RFCs discuss electronic mail but without defining it. I came across a government site that accepts confidential information but says that emails are public information, and I suppose we all know an email when we see it, but many contact forms effectively send an email and when I go to my email service provider and compose an email that I then send I compose it in a form. So I wonder if the form is irrelevant to whether an online communication is an email. Is it only email if there’s an “@” in the address? If the content is sent through headers but without an “@” does that define it as not email? I’m not really asking substantive questions here; I’m trying to identify a source that already defines the term. Thanks.