Transport rule that triggers on Sensitivity Label

Hi all, i have a problem getting 1 transport rule to work as i want it to.

I am using AIP Labels to classify documents and communication. When labeling the message as internal the message header gets modified with a "Sensitivity=Internal". I would like to use that to block the message to be sent to recepients outside the organisation.

For that i have created a transport rule that check

- if the user is from the inside

- and is sending ouside

- the message header for "Sensitivity=Internal"

- adding a block for the message with notification