Hi Greg,
To understand the issue better, I’d like to confirm if the following things are true:
1.All the emails sent from your Office 365 account through AWS to other Office 365 accounts are marked as junk emails and then delivered to the Junk folder. However, when these emails are sent to Gmail and Yahoo , then are delivered to the inbox successfully.
2. When you say “Telling Outlook to trust our own domain or a particular sender address in our domain does not help.”, do you mean you have tried to add the sender addresses or domain in the safe list, but it made no difference?
3.All the emails sent from Office 365 directly are never marked as junk.
If the answer are true, this means that all the emails are sent from Office 365 directly can be delivered to others successfully, and the issue only happened when you send emails via AWS application. This indicates the issue may be caused by the AWS application. I suggest you contact AWS support to check if other setting has been affecting email delivery .
Also, to avoid unexpected problems, I recommend you make sure all the DNS records you have added are the same with those required. Here is an article for your reference:
http://office.microsoft.com/en-us/office365-suite-help/create-dns-records-for-office-365-HA102851099.aspx?CTT=5&origin=HA102851067
If there is any misunderstanding, please feel free to let me know.
Thanks,
Vivian Wang