Block upload of documents to other office 365 tenant

MZyarah I think we are both wrong.  That file policy doesn't even apply to uploads, it's a sharing policy. 

And I may be wrong, but I believe a collaborator is defined as a user that has been given explicit access to the data.  If the user is not a collaborator, the filter would not apply. 

For the policy to work, every file shared would have to be explicitly shared to specific users.  If a file is shared without specifying the users it's intended to be shared with, the policy would not apply.

Crucially, that policy does not appear to have any baring on upload of data, because uploading a file is not defined as sharing a file.   The file policy in question is specifically a sharing policy - that means it has to be shared - upload does not trigger a sharing policy.