This kind of serious analysis is precisely what the quoted RFCs are trying to encourage.
To be maximally useful, software needs to accommodate case-sensitivity without forcing it inappropriately.
Most mail software uses configurable settings and optimized defaults to achieve this goal, but fundamentally Discourse is not Mailman - it’s using mail addresses for several purposes distinctly different than SMTP transfer operations, so there’s no reason it can’t have a distinctly different (perhaps superior) way of dealing with case issues. Insisting that you have to do it the same way as everyone else is not any better than insisting on conformance to an indefensibly narrow reading of a standards document.
Man, how did I get in an RFC interpretation argument on bOINGbOING? I do enough of that at work. The craziest one was years ago, when IBM insisted that RFC1179’s specification that “The user identification must be 31 or fewer octets” meant you could not have a zero-length user id field “because technically zero is not a number”. The fact that the entire RFC is a description of existing software that actually does permit zero-length fields moved their position not one whit!
