Exchange 2013 reject message with unintelligible reason

Hi all,

Our organization is using Exchange 2013 , Windows Server 2008 R2 OS, here is my problem:

My Exchange server reject some messages from some our customers, when I say "some" I mean :

Message with subject “RE: Vietnam return rate quote needed” from customer1@customer.com to user1@mydomain.com is ok.

Message with subject "Re: F13 Direc shipment delays - PO#E105 - E106 - Vendor Star Fashion-Vietnam" from customer1@customer.com to user1@mydomain.com is rejected.

I think it's because content filter but I already added domain customer.com into trusted list.

Set-ContentFilterConfig -BypassedSenderDomains @{add="customer.com"}

Set-SenderIdConfig -BypassedSenderDomains @{add="customer.com"}

Customer receive NDR email bounce back with reason :

Delivery has failed to these recipients or groups
'user1@mydomain.com'
Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept e-mail from certain senders, or another restriction may be preventing delivery.Diagnostic information for administrators:
Generating server: myserver.com
'user1@mydomain.com'
#< #5.7.1 smtp;550 5.7.1 TRANSPORT.RULES.RejectMessage; the message was rejected by organization policy> #SMTP#

It's weird ! I can't understand what "organization policy" rejected this message although I have some transport rules to filter spam key words but I pretty sure that none of these words exits in rejected message , as I mention above I already added customer domain into bypassed sender domain in content filter config.

I don't configure any email restriction on user1@mydomain account . One thing I doubt : user1@mydomain.com belongs to many groups , one of them restrict messages from outside only accept messages from senders inside my organization ... but the rejected message is sent directly to user1@mydomain.com , not to any group. It's probable that Exchange works incorrectly sometimes ?

It happens randomly with different messages from different customers.

Hope somebody can help . Thanks in advance.

Jack