Add it to the Actor receive orElse chaining. So user does not need to use other method than 'receive' in his code.

object Kitchen {
 // can be remote actor to offload processing
 val Sink = actor {
   case unknown => ... // handle message, log it, store in tx log or whatever
 }
}

trait UnknownMessageHandler { this: Actor =>

 def onReceive: Receive

 def handleUnknownMessages: Receive = {
   case unknown =>
     // resolve future if present, to avoid unnecessary blocking/waiting
     self.senderFuture.foreach(_.completeWithException(new
UnknownMessageException(unknown))
     Kitchen.Sink forward unknown
 }

 def receive: Receive = onReceive orElse handleUnknownMessages
}

class MyActor extends Actor with UnknownMessageHandler {
 // no need for default case
 def onReceive = {
   case msg => ...
 }
}