This, to best of my awareness, is not correct. It is not a part of RFC5322, RFC2822 or RFC822 - nothing in those RFCs establish any particular semantics of loc

This, to best of my awareness, is not correct. It is not a part of RFC5322, RFC2822 or RFC822 - nothing in those RFCs establish any particular semantics of local-part (the part before "@"), and in terms of syntax IIRC those RFCs only defines the escaping rules (backslashes, quotes, etc). RFC5233 mentions the established practice of using a separator character that splits local-part into "user" and "detail" pieces, but does not standardize any particular syntax for the separator (it cites "+" and "#" as possible examples). Moreover, this RFC is about Sieve language, and it is out of its scope to define such standard for email addresses. RFC3696 mentions "user+mailbox@example.com" but does not define any semantics, merely citing this as an example of a valid email address, then going to how it must be escaped in a mailto: URL. It is completely up to a particular provider to implement (or not) any subaddressing syntax and any logic related to it. No email specification I'm aware of makes this a requirement. (It's not like I've read every email RFC thoroughly, though, so I could be missing something.)