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 (22.214.171.124) by
DM2PR0501MB1326.namprd05.prod.outlook.com (126.96.36.199) 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 ([188.8.131.52]) by
DM2PR0501MB1327.namprd05.prod.outlook.com ([184.108.40.206]) 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 !