If it hasn't been delivered to the user's device, fine. Once it's been delivered, it shouldn't be modified. I suppose I might be OK with individual exceptions

If it hasn't been delivered to the user's device, fine. Once it's been delivered, it shouldn't be modified. I suppose I might be OK with individual exceptions to this rule, if some sensitive message was sent, after going through customer service. It certainly shouldn't be a one click to delete.