Hi,
I never saw any mail system in which the local Part ist Case sensitive!
Every Unix/Linux system (I?d say that represents the majority of mail systems) has a case sensitive local part. Most mail admins have set their systems up to normalize all users on lowercase names, but that is not and has not always been the case.
at least in every large installation LDAP User Backend's (or AD or similar) are used, and they are definitly case insensitive for the CN or UID attributes. Really nobody who cares about his systems and users uses mailboxnames which just differ in case (too much danger of confusion!)
At least sieve must reflect the same settings as the MDA, so if the MDA is set to downcase (which in my opionion is the only valuable setting) sieve must also downcase.
Sincerly, Klaus