dovecot - Nov 2005 - Ambiguous ..... problem using Dovecot v1.0.alpha4 on OpenBSD 3.8

I verified that "default_mail_env" on my dovecot configuration file
are "maildir:" correctly prefixed and ... permissions of
/var/mail/mydomain and /var/mail/mydomain/myuser are 2755 and owner is
root:mail.
Dovecot user is member of mail group.
I'm sure that isn't an authentication problem.
I've tried some days to understand and to find over internet but I've
not find a solution and now ... doesn't want to work.
Someone can help me?
_________________________________
Log output:

dovecot: Nov 03 20:20:39 Error: imap(myuser at mydomain.com): Ambiguous mail
location setting, don't know what to do with it:
/var/mail/mydomain.com/myuser (try prefixing it with mbox: or maildir:)
dovecot: Nov 03 20:20:39 Error: imap(myuser at mydomain.com): Failed to create
storage with data: /var/mail/mydomain.com/myuser
 
 
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