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

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.