Validation errors are common, but warnings are not.
I'd like to see more of "Patterns like [what you entered] are uncommon—are you sure?" instead of "Patterns like [what you entered] are not allowed—change it to proceed."
I recently implemented this using the great Mailcheck library. So if someone types "gnail.com" or "gmail.con" it detects it and we can show "Did you mean gmail.com?". If someone ignores the suggestion, fair enough. If someone purposely wants to give us a junk email, fair enough. At least we're not frustrating them needlessly.