Hi Andrew,
From the NDR, I understand that the user(let’s say A) is having issue sending emails to another internal user(let’s say B). And his emails are bounced back by the spam filtering.
To narrow down the root cause, I’d like to collect some more details:
1. When saying “a Office 365 customer who is getting bounce backs when sending emails to certain addresses”, do you mean he cannot send emails to all the internal users?
2. When did it begin to occur?
3. What’s the difference between his email and other normal email, both in terms of subjects, contents and attachments?
4. What’s the difference between this user and other users, such as location, network?
5. Do you have SMTP relay deployment in your Office 365?
Thanks,
Bernny Sun