Sendmail, qmail, exim, procmail, postfix, eudora, courier, dovecot, UWash IMAP, mercury, Microsoft Exchange, scalix, VAXmail, Novell groupwise and every other mail processing software I’ve ever adminned but can’t remember right now were distributed case-insensitive by default. That Wikipedia quote is very misleading in its use of the word “normally”.
The specifications permit case sensitivity, and several of the aforementioned can be configured for it, but far more than 99% of the Internet’s email addresses are case-insensitive. Thus the majority of the world’s software that interacts with email addresses by default is case-insensitive, and can only be configured for case-sensitivity with some difficulty.
Note that the local part of an email address can contain routing information or free form text. These addresses are all the same mailbox (I redacted the domain, though):
root@mailhost.example.com
“Maurice Meisterberger” <morrie+Root@mailhost.example.com>
“Knuckles Malone” <knuck+ROOT@mailhost.example.com>
I think that if you implement email address case-sensitivity as a default, you’re kind of bucking current best practices on a technicality - observing the letter in preference to the spirit. It should only be a discouraged option. Paul Mockapetris once advised me in a similar situation to follow Postel’s dictum of “be generous in what you accept, and conservative in what you send”. Just as email-aware programs should accept domain names terminated by a dot, but never send them, they should be capable of case sensitive comparisons, but never default to that, and always preserve case of addresses entered by humans.
Edit: fixed pointy brackets to get past meta parsing
