It doesn't have S2S inbox yet, but it has C2S outbox and was created from a client. The client controls the identity key, can send signed messages to local users, and receive messages from them. I published the code here: https://codeberg.org/silverpill/fep-ae97-client
FEP-ae97 clients are not difficult to build, and I invite everyone to do so. C2S protocol is identical to S2S protocol, except you send activities to a local outbox instead of a remote inbox, and you pull activities from local inbox instead of listening for them online. The development version of Mitra already has necessary endpoints. FEP-ae97 spec needs to be updated though, I will do that soon.