Yes, Email Addresses Are Case Sensitive.
Every email address has three parts. What comes before the '@' (the so-called "local mailbox part"), the '@' itself and what follows the '@' (the domain name).
The answer to the question whether email addresses are case sensitive — whether it matters if you type ReCipiENt@eXaMPle.cOm or RECIPIENT@EXAMPLE.COM or email@example.com — has to do with these elements of an email address.
The domain name part of an email address is case insensitive (i.e. case does not matter). The local mailbox part, however, is case sensitive. The email address ReCipiENt@eXaMPle.cOm is indeed different from firstname.lastname@example.org (but it the same as ReCipiENt@example.com).
But Case Typically Does Not Matter
Since the case sensitivity of email addresses can create a lot of confusion, interoperability problems and widespread headaches, it would be foolish to require email addresses to be typed with the correct case. Hardly any email service or ISP does enforce case sensitive email addresses, returning messages whose recipient's email address was not typed correctly (in all upper case, for example).
This means that
- it does not typically matter what case you type an email address in when you send a message
- (If the recipient did give you an email address with distinct case, preserve it, however.)
- and you should always only use lower case characters when creating a new email address to rule out any confusion.
What the Standard Says
RFC 2821, the standard that defines how email transport works, lays down the email address case sensitivity issue thus:
The local-part of a mailbox MUST BE treated as case sensitive. Therefore, SMTP implementations MUST take care to preserve the case of mailbox local-parts. Mailbox domains are not case sensitive. In particular, for some hosts the user "smith" is different from the user "Smith". However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged.