Do Not Forward not working for some emails

MichelleProper Hello again! I see what you mean as you actually can forward the message from @icloud.com (or even @gmail.com). The process isn't as seamless when using these external email providers, which is good to know. But there is no possibility to read the message, not the body or an attachment, as it's a wrapper in the email informing and directing you to the OME (Office Message Encryption) portal for authentication with the recipient email address, or a one-time pass code sent to the recipient email address.

Just to be sure I set up the scenario you described and this was the result. So the protection does work as intended, it's the forward option that isn't greyed out at the external email providers client's (but would be in @outlook.com for example).

If you don't see this behavior something is wrong and the OME and IRM settings needs to be looked at.