Overview
The University of Idaho has implemented Data Loss Prevention (DLP) rules for outbound email to reduce the potential exposure of high risk data. You may receive a Non-Delivery Receipt (NDR), or "bounce message", when you attempt to send an email message.
The message will include this wording:
A custom mail flow rule created by an admin at vandalsuidaho.onmicrosoft.com has blocked your message.
This message may contain "High Risk" data and should not be shared in email.
Example screenshot of the NDR message:

Examples of High Risk Data: HIPAA ePHI, Social Security Numbers, driver's license numbers, bank or credit account numbers. See 30.11 - University Data Classification and Standards for more detailed information on this subject.
If you do need to send high risk data, or the bounce message was incorrect (the message was not high risk), please contact your Local Support Team for assistance.