And if you go by RFC 822 you are going to have problems anyway, because it's out of date and no longer a valid description of the internet email address rules. The new RFC (RFC 2822) which superceded RFC 822 has itself been obsoleted by an even newer rfc (RFC 5322) and even RFC 5322 is partly obsoleted by RFC 6854 which replaces part of RFC 5322.

But being a domain expert is irrelevant anyway, because "valid" means "conforming to the internet rules for email addresses" rather than "being an actual email address" or "being an email address potentially allowed by the xyz.com receiving domain".