[Dovecot] can't deliver with Kerberos username

[Nikolay Shopik]

I've been trying to use Kerberos and PLAIN text file for authentication. 
I've setup deliver in postfix master.cf and make sure correct username 
are expanded but deliver is always told me "user unknown".
I can logon into mailbox using Kerberos w/o problems and can send 
emails. Looks like Dovecot lookup up passwd files and not finding users 
which are in Kerberos realm.

# 1.0.15: /etc/dovecot/dovecot.conf
log_path: /var/log/dovecot.log
protocols: imap
ssl_cert_file: /etc/postfix/new_chained.crt
ssl_key_file: /etc/postfix/mail.pem
login_dir: /var/run/dovecot/login
login_executable: /usr/lib/dovecot/imap-login
login_greeting_capability: yes
mail_location: maildir:/var/mail/store/%u
dotlock_use_excl: yes
maildir_copy_with_hardlinks: yes
auth default:
  mechanisms: PLAIN CRAM-MD5 GSSAPI
  passdb:
    driver: passwd-file
    args: /etc/dovecot/passwd
  userdb:
    driver: static
    args: uid=vmail gid=vmail home=/var/mail/store/%u
  socket:
    type: listen
    client:
      path: /var/spool/postfix/private/auth
      mode: 432
      user: postfix
      group: postfix
    master:
      path: /var/run/dovecot/auth-master
      mode: 438
      user: root
      group: root
plugin:
  sieve: /var/mail/store/%u/sieve