Hi lost_but_happy,
Would you offer us the full Non-Delivery Report (NDR) in a Private Message for our further troubleshooting?
To access to the Private Message, you can follow the steps below:
1. Please go to the details section on the right side of the community site.
2. Click Private Messages.
3. Click the subject title of the response to read the message.
In addition, I have found two similar threads for your reference.
Thread 1: https://community.office365.com/en-us/forums/146/p/80967/317863.aspx#317863
Thread 2: https://community.office365.com/en-us/forums/158/p/9071/38390.aspx#38390
If the above threads address the issue, please let us know.
Thanks,
Wilfred Ying