We have configured a mailflow rules in Exchange Online that will block any email that is labeled with a specific sensitivity label and that is sent outside the organization from being delivered. The mailflow rule actually look in the email "msip_labels" header and looks for the specific label information (ex: MSIP_Label_f777f457-ef2d-434d-81b5-0f4123455469_Enabled=true;). When found in the header, the email is blocked and a notification is sent to the sender.
This work perfectly for emails sent from Outlook.
Now that we can apply labels with Outlook on the Web (OWA), I was expecting the mailflow rule to work as well as with the Outlook client. It does not. I cannot understand why this is not working. We are applying the same label to the email in OWA as we do in Outlook. We can see in the message header that the "msip_labels" is there with the same MSIP_Label_f777f457-ef2d-434d-81b5-0f4123455469_Enabled=True; information. The only difference is that the word "True" has a capital "T" when sent from OWA (lowercase "t" when sent from Outlook) but the mailflow rule are not case sensitive anyway. Still, we did change the transport rule to also have a capital "T" but it doesn't change the result. When looking in the Message Trace of Exchange Online, the outgoing message from OWA is never analysed by the mailflow rule.
I know that this feature (sensitivity labels in Office on the web) is still in preview but I was wondering if any of you had that issue or would have an idea of what could cause the issue.