[Dovecot] Can't connect to auth server at /var/run/dovecot//auth-master

[maxxfire]

Hi,
 
 
  I'm using dovecot1.1 with postfix2.2.5; I configure dovecot.conf following the dovecot-wiki. 
  But i doesn't work. The maillog said 'Can't connect to auth server at /var/run/dovecot//auth-master: No such file or directory' when I send a mail.
 
  
my Postfix Master.cf configure:
 
dovecot   unix  -       n       n       -       -       pipe
    flags=DRhu user=vmail argv=/usr/local/libexec/dovecot/deliver -f ${sender} -d ${recipient}
 
 
  I just confused that why it needed to be authed again when postfix pipe the mail to dovecot.
 
 
Best regards!
Thanks and forgive my poor english writing.