Says RFC 5321 [1]: "The local-part of a mailbox MUST BE treated as case sensitive."
It _does_ recommend receivers treat it as case insensitive for maximum interoperability, so it is de facto insensitive, but something implementing it as case sensitive isn't broken.
It does make it broken. Broken means not working. If your software refuses an email because it's in the wrong case then that software is broken. And quoting out of an RFC is not going to make users stop complaining.
Email addresses are written i a variety of situations where preserving case is not possible. For instance on forms, or over the phone. If the IETF wants to ignore that then that's the IETF's problem, don't make it yours too.
I think it is a fair point -- when a technical standard and/or convention so vastly disagrees with common user perception, perhaps the requirement should be broadened to account for both.
On the flipside, you’re trying to tell me I should be willing to accept a lower standard than I wish to, or that I’m used to, or that has been established for decades, because of some anachronistic bureaucrat, and my response to that is a short expletive.
It _does_ recommend receivers treat it as case insensitive for maximum interoperability, so it is de facto insensitive, but something implementing it as case sensitive isn't broken.
[1] https://tools.ietf.org/html/rfc5321