[Dovecot] Dovecot auth error

[mehrdad nosrati]

Greeting

I'm newbie to Squirrelmail and just installed Dovecot in CentOS6.3.
When I try to login to the Squirrelmail, then I get the following error:

 auth: Info: passwd(myusername at myserver.com): unknown user


my config file is as follow:

# /usr/bin/doveconf -n -c /etc/dovecot/dovecot.conf


# 2.0.9: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-279.el6.x86_64 x86_64 CentOS release 6.3 (Final)
auth_debug = yes
auth_mechanisms = plain login
auth_verbose = yes
base_dir = /var/run/dovecot/
disable_plaintext_auth = no
info_log_path = /var/log/dovecot-deliver.log
log_path = /var/log/dovecot-deliver.log
mail_debug = yes
mail_location = maildir:~/MaIL
mbox_write_locks = fcntl
passdb {
  driver = passwd
}
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0666
    user = postfix
  }
}
ssl_cert = </etc/pki/dovecot/certs/dovecot.pem
ssl_key = </etc/pki/dovecot/private/dovecot.pem
userdb {
  driver = passwd
}
protocol pop3 {
  pop3_uidl_format = %08Xu%08Xv

Any help would be appreciable.

Thanks!
Mehrdad.