[Dovecot] lmtp_save_to_detail_mailbox not working?

Dovecot 2.1.6, with lmtp_save_to_detail_mailbox=yes recipient_delimiter=+

messages addressed to "user+detail@domain" are always delivered to INBOX regardless if "detail" mailbox already exists or lda_mailbox_autocreate=yes.

Anything else needed to enable lmtp_save_to_detail_mailbox feature?

Thanks,

Oscar