[rthille]

Really? By my reading RFC-5321 & RFC-5322 leaves interpretation of the local-part up to the software running on the host where the mail is delivered, but since that interpretation is up to those servers, intermediate servers must treat them as case sensitive and not make modifications to the local-part.

[FungalRaincloud]

That's my interpretation, as well. The standard is for carriers, not mailboxes. As a carrier, (or someone sending an email) you should respect case, as well as respect all of the special characters, because the server is allowed full decision power over whether those things are meaningfully used.