Way late to this post, but I've got something slightly different to say...
>> "Are email addresses case sensitive?"
Well, "It Depends..." (TM)
Some organizations actually think that's a good idea and their email servers enforce case sensitivity.
So, for those crazy places, "Yes, Emails are case sensitive."
Note: Just because a specification says you can do something does not mean it is a good idea to do so.
The principle of KISS suggests that our systems use case insensitive emails.
Whereas the Robustness principle suggests that we accept case sensitive emails.
Solution:
This would mean that if this email already exists: [email protected]
... and another user comes along and wants to use this email: [email protected]
... that our case insensitive searching logic would return a "That email already exists" error message.
Now, you have a decision to make: Is that solution adequate in your case?
If not, you could charge a convenience fee to those clients that demand support for their case sensitive emails and implement custom logic that allows the [email protected] into your system, even if [email protected] already exists.
In which case your email search/validation logic might look like something this pseudocode:
if (user.paidEmailFee) {
// case sensitive email
query = "select * from users where email LIKE ' + user.email + '"
} else {
// case insensitive email
query = "select * from users where email ILIKE ' + user.email + '"
}
This way, you are mostly enforcing case insensitivity but allowing customers to pay for this support if they are using email systems that support such nonsense.
p.s. ILIKE is a PostgreSQL keyword: http://www.postgresql.org/docs/9.2/static/functions-matching.html