The email is usually a good thing as a username because:
- the user remembers it;
- it is unique worldwide, thus simplifying the management of collisions (if one user wants to use the same login name as another, then one of them did a mistake);
- it can be coupled with an "email verification" system which is convenient if you want the server to be able to contact users in case of emergency.
Nominally, at least the right half of the email address (the domain name, after the "@" sign) is case-insensitive, so you should normalize that part to lowercase, which is easy, since it is supposed to be a valid domain name, hence limited to a subset of ASCII (Note: you will want to take care to use what .NET calls the invariant culture, and Java terms the root locale; otherwise, your code will break in Turkey).
For what is on the left, case sensitivity depends on the receiving site. Most sites are case insensitive for that part too, and it seems "reasonable" to do lowercase normalization, because it is improbable that a given site is both case sensitive and uses case to distinguish between distinct people (i.e. that
BOB@example.com are both valid addresses for two different Bobs). Thus, I suggest lowercase normalization of the whole address for comparison purposes (i.e. to decide which user we are talking about); but keep the address "as is" if you ever want to send back an email to the user, or even if you want to show it to the user (e.g. as a "Welcome, Bob@example.com" banner -- Bob might be quite fond of his uppercase 'B').
About the "+" sign: From your point of view, that's part of the address. This "+" is handled on some sites as a way for each user to generate a lot of functionally equivalent addresses: Bob will be able to use
email@example.com... all emails sent to any of these addresses end in Bob's mailbox, but, in the eye of Bob, they still are distinct addresses which Bob types as such. Bob expects the addresses to be considered distinct. So your handling of the "+" depends on what you really want:
- If you just want a unique "login name" so that management of collisions is easy, then leave the address "as is"; don't do anything special with the "+".
- If you want to enforce uniqueness of accounts per human user (i.e. you don't want Bob to be able to create one million distinct accounts), you may want to remove characters from the "+" sign to the "@" sign, there again for comparison purposes. But don't believe this rule will deter most Bobs; obtaining zillions of email addresses without a "+" is easy and cheap (the ultimate way being to buy a domain and rent a server to host it).
Summary: keep the address as entered at registration time and use it "as is" for display and for sending emails. For comparisons (i.e. locating the user entry in the table of users, e.g. upon login), normalize the email to lowercase (with an invariant culture).