Email and Case Sensitivity
“Can an email address be case-sensitive? The answer is ‘yes…if you’re evil.’” — Dylan Beattie, Email vs Capitalism, or, Why We Can't Have Nice Things
Although in practice these days an email address is case-insensitive, strictly speaking—that is, according to the standard—it doesn’t have to be. As Beattie notes in the talk linked above, in the early days most email travelled on Unix systems, on which alice, Alice, and alicE are all equally valid, and, importantly, separate users. Mail administrators, even today, could theoretically enforce this if they had a reason.
Any system that includes an SMTP server supporting mail relaying or delivery MUST support the reserved mailbox “postmaster” as a case-insensitive local name.