Configuration

Enable the auth service in /etc/dovecot/conf.d/10-master.conf:

service auth {
  unix_listener auth-userdb {
  }

  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
    user = postfix
    group = postfix
  }

  unix_listener /var/spool/postfix/auth-master {
    mode = 0660
    user = vmail
    group = postfix
  }

  user root
}

Edit /etc/postfix/main.cf:

broken_sasl_auth_clients = no
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
smtpd_sasl_local_domain =
smtpd_sasl_authenticated_header = no

Reload the Dovecot configuration to enable Dovecot SASL:

/etc/init.d/dovecot reload

Reload the Postfix configuration to use Dovecot SASL:

/etc/init.d/postfix reload

Check if Dovecot is listening on /var/spool/postfix/private/auth:

netstat -anp | grep dovecot