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?