do you treat Email Addresses as case-insensitive?

This is a Best Practices question about the use of case-sensitive email addresses.

Logic would suggest that even though the rfc does not specify that the local-part of an address should be case-insensitive , best practices would suggest otherwise making it effectively so for any and all reputable ESPs.

Per rfc5321 

   As used in this specification, an "address" is a character string
   that identifies a user to whom mail will be sent or a location into
   which mail will be deposited.  The term "mailbox" refers to that
   depository.  The two terms are typically used interchangeably unless
   the distinction between the location in which mail is placed (the
   mailbox) and a reference to it (the address) is important.  An
   address normally consists of user and domain specifications.  The
   standard mailbox naming convention is defined to be
   "local-part@domain"; contemporary usage permits a much broader set of
   applications than simple "user names".  Consequently, and due to a
   long history of problems when intermediate hosts have attempted to  

   optimize transport by modifying them, the local-part MUST be
   interpreted and assigned semantics only by the host specified in the
   domain part of the address.

Thus strictly speaking the local-part of the address could be case-sensitive.
It would seem to me that common and best practice would suggest it is not, and should not, be so.

I know Sugar uses two fields email_address and email_address_caps, one to store the address as entered, the other for filtering and determining if the email address already exists, which is totally reasonable in my opinion, and suggests an assumption of case-insensitive assignment of email addresses to individuals.

So my question is, has anyone ever come across an ESP who actually uses a case-sensitive local-part in their email addresses? 

thanks,

FrancescaS