auto: mdbox is automatically recognized as sdbox

[??????]

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(q...@a.com)<30883&gt;<+nFkVxul5oF/AAAB&gt;: Debug: 
Mailbox INBOX: Mailbox opened because: SELECT
May 08 12:22:15 mail imap(q...@a.com)<30883&gt;<+nFkVxul5oF/AAAB&gt;: 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(q...@a.com)<30883&gt;<+nFkVxul5oF/AAAB&gt;: 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
&nbsp;&nbsp; &nbsp;
&nbsp; &nbsp;&nbsp;I am confused why mdbox and sdbox cannot be recognized 
automatically or there are same problem with my test ?? &nbsp; Thanks