Hi Mike,
Based on your description, I would like to confirm the following information:
1. Is the parent organization using the same domain with you?
2. Can you receive the other emails from both internal and external accounts (such as Hotmail)?
To troubleshoot the issue, you can refer to the following link:
http://community.office365.com/en-us/wikis/exchange/inbound-mail-flow.aspx
In addition, would you please give me entire full non-delivery report (NDR) for further troubleshooting. To private your message, I have sent a private message with a subject of <Incoming mail from listserv being rejected.> to you. To access the private message:
-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
Thanks,
Anna Shi