We use Postfix on our CentOS 6 machine, having the following configuration. We use PHP's mail() function to send rudimentary password reset emails, but there is a problem.

As you will see, mydomain and myhostname is correctly set, afaik.

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
command_directory = /usr/sbin
config_directory = /etc/postfix
daemon_directory = /usr/libexec/postfix
data_directory = /var/lib/postfix
debug_peer_level = 2
html_directory = no
inet_interfaces = localhost
inet_protocols = all
mail_owner = postfix
mailq_path = /usr/bin/mailq.postfix
manpage_directory = /usr/share/man
mydestination = $myhostname, localhost.$mydomain, localhost
mydomain = ***.ch
myhostname = test.***.ch
newaliases_path = /usr/bin/newaliases.postfix
queue_directory = /var/spool/postfix
readme_directory = /usr/share/doc/postfix-2.6.6/README_FILES
sample_directory = /usr/share/doc/postfix-2.6.6/samples
sendmail_path = /usr/sbin/sendmail.postfix
setgid_group = postdrop
unknown_local_recipient_reject_code = 550

Now this is the stuff that is in the /var/log/maillog of Postfix upon sending an email to ***.***@***.ch, with ***.ch being the same domain our sending server test.***.ch is on:

Dec 13 16:55:06 R12X0210 postfix/pickup[6831]: E6D6311406AB: uid=48 from=<apache>
Dec 13 16:55:06 R12X0210 postfix/cleanup[6839]: E6D6311406AB: message-id=<20121213155506.E6D6311406AB@test.***.ch>
Dec 13 16:55:07 R12X0210 postfix/qmgr[6832]: E6D6311406AB: from=<apache@test.***.ch>, size=1276, nrcpt=1 (queue active)
Dec 13 16:55:52 R12X0210 postfix/smtp[6841]: E6D6311406AB: to=<***.***@***.ch>, relay=mail.***.ch[**.**.249.3]:25, delay=46, delays=0.18/0/21/24, dsn=5.0.0, status=bounced (host mail.***.ch[**.**.249.3] said: 550 Sender Rejected (in reply to RCPT TO command))
Dec 13 16:55:52 R12X0210 postfix/cleanup[6839]: 8562C11406AC: message-id=<20121213155552.8562C11406AC@test.***.ch>
Dec 13 16:55:52 R12X0210 postfix/bounce[6848]: E6D6311406AB: sender non-delivery notification: 8562C11406AC
Dec 13 16:55:52 R12X0210 postfix/qmgr[6832]: 8562C11406AC: from=<>, size=3065, nrcpt=1 (queue active)
Dec 13 16:55:52 R12X0210 postfix/qmgr[6832]: E6D6311406AB: removed
Dec 13 16:55:52 R12X0210 postfix/local[6850]: 8562C11406AC: to=<root@test.***.ch>, orig_to=<apache@test.***.ch>, relay=local, delay=0.13, delays=0.07/0/0/0.05, dsn=2.0.0, status=sent (delivered to mailbox)
Dec 13 16:55:52 R12X0210 postfix/qmgr[6832]: 8562C11406AC: removed

So the receiving server rejects the sender (line 4 of log output). We have tested it with one other recipient and it worked, so this problem might be completely unrelated to our settings, but related to the recipient.

Still, with this question, I want to make sure we're not making an obvious misconfiguration on our side.