I thought email addresses were case folded on the right hand side and site dependent on the left hand side.
The right hand side is more or less forced by the rules of DNS.
As for the left hand side, if I run the email for a site, can't I decide whether to deliver ABC@ and abc@ both to the same mailbox or to different mailboxes? And can't someone else make a different decision for their site?
If a site administrator does not have the prerogative to decide this, what rule prevents them? (And if there is such a rule, can you rely on it being enforced?)
Of course, but when processing an arbitrary email address, which will almost always be not on your site, you MUST treat the left hand side as case-sensitive (unless you have knowledge about that email domain).
site dependent means that when given an address you must treat the lhs as case sensitive. To do otherwise will mean that you've potentially broken the address and can no longer properly use it.
The right hand side is more or less forced by the rules of DNS.
As for the left hand side, if I run the email for a site, can't I decide whether to deliver ABC@ and abc@ both to the same mailbox or to different mailboxes? And can't someone else make a different decision for their site?
If a site administrator does not have the prerogative to decide this, what rule prevents them? (And if there is such a rule, can you rely on it being enforced?)