Currenlty only Mailjet Support can set ReturnPath. We have hundreds of subaccounts and Mailjet involement is needed for the return Path Setting.
In order to reduce effort on Mailjet side and also to help us in managing our own subaccounts better, it would be helpful that the Master-Account user is able to do the reurn path setting. If adding this to the frontend takes to much time, it would be enough as a first step, just to have an API for setting the returnpath.