Delivery case sensitive?

Hi there,

I'm using postfix on Mac OS X 10.3 (Panther) and wondering why I get
these errors, when sending a Mail from User 'guest' to 'USER':

Jan 8 14:50:52 localhost postfix/pickup[3662]: 9AAB8A98B2: uid=502
from=<guest>
Jan 8 14:50:52 localhost postfix/cleanup[3663]: 9AAB8A98B2:
message-id=<20040108135052.9AAB8A98B2@host.domain.org>
Jan 8 14:50:52 localhost postfix/qmgr[3665]: 9AAB8A98B2:
from=<guest@domain.org>, size=316, nrcpt=1 (queue active)
Jan 8 14:50:52 localhost postfix/local[3667]: 9AAB8A98B2:
to=<USER@host.domain.org>, relay=local, delay=0, status=bounced (unknown
user: "user")
Jan 8 14:50:52 localhost postfix/cleanup[3663]: BE54BA98B4:
message-id=<20040108135052.BE54BA98B4@host.domain.org>
Jan 8 14:50:52 localhost postfix/qmgr[3665]: BE54BA98B4: from=<>,
size=1883, nrcpt=1 (queue active)
Jan 8 14:50:52 localhost postfix/smtp[3689]: BE54BA98B4:
to=<guest@domain.org>, relay=none, delay=0, status=bounced (Name service
error for name=domain.org type=A: Host found but no data record of
requested type)

(I've changed domain and user names for better reading)

Is it possible, that postfix handles mail adresses case sensitive?

Mails from 'USER' (capitals!) to 'guest" (lower case!) are no problem.
But Mails in the other direction will not get delivered (log above).

Do you have any idea what the problem might be?

Thanks in advance
reiner

PS
$ postconf -n
alias_database = hash:/etc/aliases
alias_maps = netinfo:/aliases
command_directory = /usr/sbin
config_directory = /etc/postfix
daemon_directory = /usr/libexec/postfix
debug_peer_level = 2
inet_interfaces = localhost
mail_owner = postfix
mailq_path = /usr/bin/mailq
manpage_directory = /usr/share/man
mydestination = $myhostname, localhost.$mydomain
mydomain = domain.org
mydomain_fallback = localhost
myhostname = host.domain.org
mynetworks = 192.168.10.0/28, 127.0.0.0/8
mynetworks_style = host
myorigin = $mydomain
newaliases_path = /usr/bin/newaliases
queue_directory = /private/var/spool/postfix
readme_directory = /usr/share/doc/postfix
relay_domains = $mydestination
sample_directory = /usr/share/doc/postfix/examples
sendmail_path = /usr/sbin/sendmail
setgid_group = postdrop
unknown_local_recipient_reject_code = 450