Hi Havard,
According to the NDR (Non delivery report), it seems that the emails are sent from the High Risk Delivery Pool (HRDP). If the Exchange Online server used High Risk Delivery Pool to deliver the email, it indicates the Exchange Online server regards the mail
as a potential spam, or the customer's email system has been compromised by malware or a malicious spam attack.
The use of a dedicated high risk delivery pool helps ensure that the normal outbound pool is only sending messages that are known to be of a high-quality. The possibility of the high risk delivery pool being placed on a blocked list remains a risk. This
is by design. This secondary IP pool helps to reduce the probability of the normal outbound-IP pool being added to a blocked list.
However, to definitely understand the situation and confirm whether the emails are really sent from the HRDP, could you please provide the following information?
1. Are all the outbound emails rejected with the same NDR from your organization?
2. Try to send some Plain Text messages, check the result. And confirm whether they are sent by the High Risk Delivery Pool or not.
3. Create a new Office 365 user account (which means an account/email address has never sent emails yet) and do the above test again.
4. I have sent you a private message to collect the detailed NDR (Non delivery report), you can access the private message via the following steps:
- Go to "Your details" section on the right side of the community site.
- Click Private messages.
- Click the subject title of the response to read the message.
Regards,
Frank
