> the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address.
In fact, that RFC specifically calls out that interpreting the `+` on non-local addresses is likely wrong:
> NOTE: Because the encoding of detailed addresses are site and/or implementation specific, using the subaddress extension on foreign addresses (such as the envelope "from" address or originator header fields) may lead to inconsistent or incorrect results.
EDIT to address your second point:
> After all - if they're being interpreted "as" user IDs, rather than as emails, does that really breach the RFC?
Well, technically no, the RFC is about SMTP so if you're not writing an SMTP implementation, you're not breaching it.
But RFCs aren't the law, so whether you're technically breaching it isn't really what's relevant. What _is_ relevant is that a system that treats [email protected] the same as [email protected] is making assumptions about how email works that contradict the RFCs that define how email works. Whether that's a useful thing to do in practice is an engineering decision with tradeoffs. E.g., it's probably fine to assume it for a whitelisted set of domains where you know it to be true, like gmail.
Actually, no. To the best of my knowledge (and I'd be delighted to be corrected!), that's merely a convention that lots of providers (including GMail) conform to, but it's not part of the RFC or standards.
Don't get me wrong - it irritates me when that very-common behaviour isn't supported (and, at the very least, `+` shouldn't be considered an illegal character). But it's also technically-not-wrong to consider `[email protected]` as different from `[email protected]`.