Relay access denied Problem

[izadi60]

Dear Friends, I'm using "zcs-6.0.8_GA_2661.RHEL5.20100820051652" on CentOS 5.5. I have 5 domains.
Some of Emails From a special domain to users are rejected.For Example An Email is sent to 6 users ,4 of them are delivered but 2 of them are rejected.the error is : reject: RCPT from xxx.xxx[IP]: 554 5.7.1 <user@domain>: Relay access denied; from=<user@specialdomain> to=<user@domain> proto=ESMTP helo=<xxx.xxx>
I added the special domain to whitelist and also add its IP to my networks, but no effect.
any suggestion?
thanks

[phoenix]

.....and also add its IP to my networks, but no effect.

Do not do that under any circumstances, you could end up being an open relay for that server.

This problem has been covered many times in the forums, search for the error message and you should find the answer. You should also post the error messages before and after that, posting one line out of context isn't much help. I'll also move this to the correct forum.

[izadi60]

Thanks alot,
of course I searched before I posted the problem here.but that answeres could not help me.
and also there is not any thing weird before and after that
any suggestion? pls

[phoenix]

Thanks alot,
of course I searched before I posted the problem here.but that answeres could not help me.

What have you tried so far? It would help if you told us otherwise we'll be going over the same answers.

and also there is not any thing weird before and after that

It would still help if, in future, you posted them for others to see and make that judgement.

[izadi60]

this is log file:
Nov 28 14:04:49 zmail postfix/smtpd[29893]: connect from xxx.xxx[IP]
Nov 28 14:04:53 zmail postfix/smtpd[29893]: NOQUEUE: reject: RCPT from xxx.xxx[IP]: 554 5.7.1

<user@mydomain>: Relay access denied; from=<user@specialdomain> to=<user@mydomain> proto=ESMTP helo=<xxx.xxx>
Nov 28 14:04:53 zmail postfix/smtpd[29893]: disconnect from xxx.xxx[IP]
..
I read the document bellow but I think everything is OK with my config
ZimbraMtaMyNetworks - Zimbra :: Wiki

also this is my main.cf:
permitted_senders_list = check_sender_access hash:/opt/zimbra/postfix/conf/permitted_senders, reject
smtpd_restriction_classes = permitted_senders_list
smtpd_recipient_restrictions = check_recipient_access hash:/opt/zimbra/postfix/conf/protected_recipients, reject_non_fqdn_recipient, permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination, reject_unlisted_recipient, permit

Someone suggested to create cert again,I did it too .But it doesn't work..
It happens just for some users sometimes.
thannks