Just because [email protected] is valid according to the RFC does not mean xyz.com is an existing domain name with a mail server setup that an account 'myname', let alone that user who inputted actually has access to that account. Just send a validation email

You are confusing "valid email address" with "a valid email address, functioning mail server and user with appropriate credentials to access the system that processes email for the supplied email address."

What you quoted was "valid user's email address", not "valid email address". While "valid email address" is arguable, a "valid user"'s email address is more clearly about whether it accepts mail. :)

A "valid" email address does not have to mean a "well-formed" email address.

Without getting too existential what else could "valid" possibly mean.

One that someone is using to accept mail.