Inbox must allow other people to access it to send you notes or DMs. If Citrine is only running on your phone, no one will be able to access it. Tor opens it up to the world. If you are not using something it open it up, you will never receive a DM.  

 Let's see if I get this correctly: if I'm running citrine + TOR relay for DMs, others should be running TOR, plus they should be using my relay to write to the relay and read from the relay?  

 Correct. If that is the only relay there, then people must use Tor.