postfix aliases

hi

i installed postfix, and everything is ok, except working aliases.
i onley get this error :

Mar 23 17:04:05 inetsrv postfix/lmtp[8982]: B319C1F97F:
to=<demo@mydomain.com>,
relay=/var/run/cyrus/socket/lmtp[/var/run/cyrus/socket/lmtp], delay=1,
status=bounced (host
/var/run/cyrus/socket/lmtp[/var/run/cyrus/socket/lmtp] said: 550-Mailbox
unknown. Either there is no mailbox associated with this 550-name or
you do not have authorization to see it. 550 5.1.1 User unknown (in
reply to RCPT TO command))


the error seems to come from cyrus, but i think it's because postfix
send the mail with applying the aliasing.

i also got this in main.cf
alias_maps = hash:/etc/postfix/aliases


and that in /etc/postfix/aliases :
demo: kroyer

i use this command to create the database
postalias hash:/etc/postfix/aliases

my users database are not store in standard unix accounts files, but in
an ldap directory.
cyrus, and amavisd are both using this directory. cyrus for
authentication, and amavis for user filtring.

so my question is, why does not mails to demo@mydomain.com going tp user
kroyer@mydomain.com.

am i forgotting something ?

thanks for any help