Short answer: yes. Long answer: The part after the '@' (the domain name) is *not* case sensitive. However, the user's mailbox (the part before the '@') is, by the definition of the specification, case sensitive. While most ISPs do not enforce the case sensitivity, to be perfectly compatible you should allow both upper and lower case for the mailbox name.
This is what RFC 2821, the standard that defines how email transport works, says about it:
"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."
