MIP SDK cannot read file labels if a message was encrypted by Outlook Classic.

Hi Oleg, I would just dowgrade the document to a label without encryption using set label and leave a justification as a good practise. This will remove protection as well. Further you will have the actions logged locally which you can upload to az-loganalytics for compliance reasons. Your compliance dept. may need to know who removed the lock to access crown-jewels.
If you remove protection and still remain the declaration eg. "secret" would be at least strange or give a false sense of security.

Or use a transport rule on EXO, this will possibly do the job as well.

On a second read, I am not sure if Outlook requires a Label or not? If Outlook does not us a Label and just "encrypt" is used - IMHO Outlook applies OME not MIP encryption based on label. Therefore the message would not posess a label, it would be just encypted. Outlook does not inherit the label and its settings by default. Once there was a optional setting which enforced the higher order of labels to be applied to messages.

Regards Max