[Dovecot] Ambiguous ..... problem using Dovecot v1.0.alpha4 on OpenBSD 3.8

[snoop at email.it]

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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