Like many websites, we use email addresses as user identifiers for logins.
RFC 5321 [#2.3.11] states that the local parts of email addresses:
MUST be interpreted and assigned semantics only by the host specified in the domain part of the address
This means that applications processing email addresses can't second guess, for example, the way that GMail will ignore dots when determining usernames. It also means that applications must treat email address local-parts as case sensitive, since that's how the local part is specified.
However users are stupid, and in a recent case a user who had signed up with an uppercase email address (and had successfully received and clicked the verification email) found they were unable to log in because they were now using the lowercase format of their email address.
I've never come across an email server which enforced case-sensitivity on its inboxes. I don't doubt that some exist somewhere, but I'm questioning whether there isn't a bigger benefit to dropping case-sensitivity for our email usernames than there are problems keeping it.
I wouldn't go so far as to emulate GMail's dot processing, or any kind of plus-addressing, because those don't seem to be as ubiquitous or as automatically-assumed as case-sensitivity. However, I'm aware it's not a black and white scenario so I'm interested if anyone knows how widespread case-sensitive email addresses actually are.