AIP sent file out of Microsoft cannot open

Good morning,

I'm lately on a project to present AIP solution and I stumbled upon an issue which I cannot find the exit.

When sending an email with a protected document to an external user that is out of Microsoft domain, the document won't open.

Know that I used 2 different label to protect and send the file:

- Administrator choice to add the partner email in the label

- User's choice to add the partner email when classifying and protecting the file.

We may have skipped something in the configuration and are not aware of it, but cannot put our finger on it.

If you have any help or information about the issue, that would be great.

Thank you