Hi:   I did a simple test with Dovecot-2.3.9:            First step : A test account uses mdbox format to store emails with 'mail_location = mdbox:%h/%Ld/%Ln:ALT=%h/altstorage/%Ld/%Ln',           Second step: I adjust 'mail_location = auto:'  and dovecot restart           Third step: telnet 127.0.0.1 143  then log in and fetch emails.     When I fetch email data , the log shows that the storage format is automatically recognized as sdbox and index is Rebuild, as the result, all emails is lost：

May 08 12:22:15 mail imap(q2@a.com)<30883><+nFkVxul5oF/AAAB>: Debug: Mailbox INBOX: Mailbox opened because: SELECT May 08 12:22:15 mail imap(q2@a.com)<30883><+nFkVxul5oF/AAAB>: Warning: sdbox /srv/vmail/a.com/q2/mailboxes/INBOX/dbox-Mails: Rebuilding index May 08 12:23:12 mail auth-worker(30882): Debug: conn unix:auth-worker (pid=29526,uid=12348): Disconnected: Connection closed (fd=-1) May 08 12:24:19 mail imap(q2@a.com)<30883><+nFkVxul5oF/AAAB>: Info: Logged out in=50 out=881 deleted=0 expunged=0 trashed=0 hdr_count=0 hdr_bytes=0 body_count=0 body_bytes=0          I am confused why mdbox and sdbox cannot be recognized automatically or there are same problem with my test ??   Thanks