[Dovecot] lmtp lowercase/uppercase mailbox problem

[johnw]

after upgrade to 2.0.x, and change dovecot-lda to dovecot-lmtp as mail
deliver.
i find the lowercase/uppercase mail box problem,
i see some mail is delivery to uppercase mail box,
(my mail box is /var/vmail/johnw, now i see some mail in
/var/vmail/JOHNW, i use maildir)
it did not happen before. (dovecot 1.2.x with dovecot-lda)

anyone can point me which config/setting, should i looking for?

i use postfix as smtp server, and i use virtual mailuser.
both user info is stored in mysql database.

please help, and thank you.