Qmail doesn't distinguish between usernames on the basis of case. It lowercases everything and doesn't deliver to users with uppercase letters in their usernames:
http://www.lifewithqmail.org/lwq.html#uppercase-usernames
Thus it seems reasonable that the comparison should also be case-insensitive.
Alas, the whole world is not using qmail. I looked up the proper RFC now; it's RFC-2822, section 3.4.1, which states:
The local-part portion is a domain dependent string. In addresses, it is simply interpreted on the particular host as a name of a particular mailbox.
What this means is that case, quoting, even allowed characters, is completely up to the domain in question to define. The chapter in the O'Reilly RegEx book which discusses regex's and e-mail addresses has some very scary examples of perfectly legal (though admittedly insane) e-mail addresses.
John
