Are mailboxes case sensative?

[Quinton McCombs]

Hello.

I have recently setup the cyrus imap 2.1.12 server on SuSE linux 8.0
with Postfix as my MTA.  I have noticed that some of my mail messages
are getting bounced because of what appears to be a case issue with the
mailbox name.

This is one of the error messages:
<HBuffington at nequalsone.com>: data format error. Command output:
HBuffington: Mailbox does not exist

If I send the message again using hbuffington at nequalsone.com, it is
delivered correctly.

Is this an issue with postfix or cyrus?