Staged migration O365, mail bouncing back for non-migrated users 550 5.7.1

Hi,

I am currently in the middle of a staged migration, and I am facing a strange issue : some mails are bounced back with an NDR code 550 5.7.1 : Message rejected due to content restrictions. What I don't understand is that it the generating server is an Outlook online server, but it only affects non-migrated users (even when both the sender and recipients are non-migrated) :

"Received: from DM2PR0501MB1327.namprd05.prod.outlook.com (25.160.130.156) by
 DM2PR0501MB1326.namprd05.prod.outlook.com (25.160.130.155) with Microsoft
 SMTP Server (TLS) id 15.0.1024.12; Mon, 8 Sep 2014 19:03:09 +0000
Received: from DM2PR0501MB1327.namprd05.prod.outlook.com ([25.160.130.156]) by
 DM2PR0501MB1327.namprd05.prod.outlook.com ([25.160.130.156]) with mapi id"

The MX records are still pointing to my On-Premise exchange, and I didn't make any change into the default behavior of the anti-spam policies in O365. Any idea on why mails between non migrated users are filtered by Exchange Online ?

Thank you !
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maxalarieAsked:
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Vasil Michev (MVP)Commented:
Sorry, can you provide a bit more details? What do you mean it's affecting only non-migrated users, why are messages sent between non-migrated users reaching O365 in the first place? Are you using EOP as the filtering platform only, without using Exchange Online (yet)?

"Message rejected due to content restrictions" definitely looks like something is wrong with the EOP settings, are you certain they are the default ones? You can create a new one specific for the domain in question if needed. You can also create an 'allow' rule and/or add your IPs to the IP allow list.
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maxalarieAuthor Commented:
Hi Vasil,

Thanks for answering. Why messages sent between non-migrated users are reaching O365 is what I'm trying to figure out actually.  I did not configure at all EOP to use it as a filtering platform, which puzzle me even more. I will try you suggestion to  allow affected domains.

Thank you !
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Vasil Michev (MVP)Commented:
Make sure to check the targetaddress attribute on the affected mailboxes, it might explain the funny mailflow (it's what the staged migration wizard would change to make sure the migrated users continue to receive mail).
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