[Dovecot] Virtual-Auth-Problem "user unknown"

[strace at letras.net]

After setting up dovecot 1.0beta-2 on a debian(etch) system
I can well login as local user but not vor virtual domains.

When I try to login via POP3 as via IMAP connects (here with 
telnet from localhost), I get a strange output "user unknown":

-------------------------
/var/log/dovecot
-------------------------
2006-02-28 01:07:38 Info: auth(default): client in: AUTH        1       PLAIN   service=POP3 secured lip=127.0.0.1   rip=127.0.0.1   resp=AHh5ekBub3JhLmxpAGRrc2RrU20=
2006-02-28 01:07:38 Info: auth(default): passwd-file(xyz at nora.li,127.0.0.1): unknown user
2006-02-28 01:07:39 Info: auth(default): client out: FAIL       1       user=xyz at nora.li
2006-02-28 01:07:40 Info: pop3-login: Aborted login: user=<xyz at nora.li>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, secured

The abobe test user xyz at nora.li exists an receives
valid email via postfix (see below in ls -laR).

My configuration (user dovecot exists with uid 119):

-------------------------
/etc/dovecot/dovecot.conf
-------------------------
base_dir = /var/run/dovecot/
protocols = imap imaps pop3 pop3s
listen = *
ssl_disable = no
ssl_cert_file = /etc/ssl/certs/dovecot.pem
ssl_key_file = /etc/ssl/private/dovecot.pem
disable_plaintext_auth = no
info_log_path = /var/log/dovecot
log_timestamp = "%Y-%m-%d %H:%M:%S "
syslog_facility = mail
login_dir = /var/run/dovecot/login
login_chroot = yes
login_user = dovecot
login_process_per_connection = yes
login_processes_count = 3
login_max_processes_count = 128
login_max_logging_users = 256
login_greeting = Dovecot ready.
login_log_format_elements = user=<%u> method=%m rip=%r lip=%l %c
login_log_format = %$: %s
max_mail_processes = 1024
verbose_proctitle = yes
verbose_ssl = yes
first_valid_uid = 200
mail_extra_groups = mail
mail_debug = yes
default_mail_env = maildir:/var/spool/vmail/%d/%n
mail_drop_priv_before_exec = yes
protocol imap {
  mail_use_modules = yes
  mail_modules = /usr/lib/dovecot/modules/imap
  imap_client_workarounds = outlook-idle tb-extra-mailbox-sep
}
protocol pop3 {
  pop3_uidl_format = %08Xu%08Xv
  mail_use_modules = no
  mail_modules = /usr/lib/dovecot/modules/pop3
  pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
}
auth_executable = /usr/lib/dovecot/dovecot-auth
auth_verbose = yes
auth_debug = yes
auth_debug_passwords = yes
auth default {
  mechanisms = plain login
  passdb passwd-file {
    args = /etc/vmail/passwd.%d
  }
  userdb static {
    args = uid=200 gid=200 home=/var/spool/vmail/%d/%n
  }
user = root
}

-------------------------
/etc/passwd
-------------------------
vmail:x:200:200::/home/vmail:/bin/sh

- that's the user postfix brings mail in

-------------------------
/etc/group
-------------------------
vmail:x:200:

----------------------------------------
`ls -laR /var/spool/mail/nora.li/xyz
----------------------------------------
/var/spool/vmail/nora.li/xyz:
total 20
drwxr-sr-x  5 vmail vmail 4096 2006-02-27 17:32 .
drwxr-sr-x  4 vmail vmail 4096 2006-02-27 17:32 ..
drwx------  2 vmail vmail 4096 2006-02-27 17:32 cur
drwx------  2 vmail vmail 4096 2006-02-27 18:30 new
drwx------  2 vmail vmail 4096 2006-02-27 18:30 tmp

/var/spool/vmail/nora.li/xyz/cur:
total 8
drwx------  2 vmail vmail 4096 2006-02-27 17:32 .
drwxr-sr-x  5 vmail vmail 4096 2006-02-27 17:32 ..

/var/spool/vmail/nora.li/xyz/new:
total 16
drwx------  2 vmail vmail 4096 2006-02-27 18:30 .
drwxr-sr-x  5 vmail vmail 4096 2006-02-27 17:32 ..
-rw-------  1 vmail vmail  813 2006-02-27 18:14 1141060448.V303I33f01M21995.tapas.letras.de
-rw-------  1 vmail vmail  813 2006-02-27 18:30 1141061439.V303I33f4dM13454.tapas.letras.de

/var/spool/vmail/nora.li/xyz/tmp:
total 8
drwx------  2 vmail vmail 4096 2006-02-27 18:30 .
drwxr-sr-x  5 vmail vmail 4096 2006-02-27 17:32 ..

==============================================================

Any hint will be highly appreciated. Thanx in advance,

Werner