Hyperledger Composer Playground Business Logic relationship - blockchain

Question

I'm tasked with learning about HyperLedger Fabric, and I'm supposed to build a business network that has many companies (participants), each company has many Outboxes (also Participants?).
Each Outbox can write Messages(Assets) to the Blockchain.
However, only the company to which the Outbox belongs should be allowed to view/edit etc the message.
So far I've managed to get most of this running, but as of now I still can create new transactions where the message belongs to company 1 and is sent by Outbox 2, which should belong to company 2.
How do I resolve this so that it is impossible to create a Message that belongs to a company 1 and simultaneously to an outbox 2 that itself doesn't belong to company 1?
asset Message identified by messageId {
o String messageId
o String text
--> Company writerCompany
--> App writerApp
}
abstract participant Member identified by memberId{
o String memberId
o String memberName
}
participant App identified by appId{
o String appId
--> Company company
}
participant Company extends Member{
}
participant Auditor extends Member{
}
transaction Write {
--> Message message
--> App writerApp
}
function Write(write) {
write.message.writerApp = write.writerApp;
return getAssetRegistry('org.acme.mynetwork.Message')
.then(function (assetRegistry) {
return assetRegistry.update(write.message);
});
}