I have a very simple question, but I haven't found anything on the Internet (maybe I don't know how to search for it).
If I deploy an actor (actorSystem.actorOf ...) and I send a message to it immediately, if the Actor hasn't been deployed yet will the messages be enqueued in a "special" queue or will the messages be sent to DeadLetters?
Have a look at the bottom of the mailbox documentation. Your guess is correct that messages are stored in a special queue until the mailbox is ready.
In order to make system.actorOf both synchronous and non-blocking while keeping the return type ActorRef (and the semantics that the returned ref is fully functional), special handling takes place for this case. Behind the scenes, a hollow kind of actor reference is constructed, which is sent to the system’s guardian actor who actually creates the actor and its context and puts those inside the reference. Until that has happened, messages sent to the ActorRef will be queued locally, and only upon swapping the real filling in will they be transferred into the real mailbox.