[Dovecot] configure lmtp to deliver to email addresses case insensitively

[Adrian Zaugg]

Dear List

Using dovecot 2.1.7 with LMTP and exim4 I want to accept local parts 
regardless of their case.

Exim does all virtual alias handling and delivers the messages to 
dovecot LMTP addressed to the right mailbox name. This works well except 
for addresses which do not need to be resolved by exim and which do have 
the "wrong" case in the local part.

For example: There is a mailbox and a user on the mailsystem 
"example.org" called "user". Mails to "user at example.org" get delivered 
just as they should. If an incoming message is addressed to 
"USER at example.org" with upper case local part, exim passes the message 
to LMTP with the case unaltered (RFC 2821 conform), but LMTP fails with:
     LMTP error after RCPT TO:<USER at example.org>:
     550 5.1.1 <USER at example.org> User doesn't exist:
     USER at example.org

How can I tell dovecot to deliver "USER" to the mailbox "user" aswell?

Thank you for your help.

Best regards, Adrian.

(I have tried to use %Ln and %Lu in mail_location setting with no success.)