Akka docs, 2.2.7: "Once an actor terminates, i.e. fails in a way which is not handled by a restart, stops itself or is stopped by its supervisor, it will free up its resources, draining all remaining messages from its mailbox into the system’s “dead letter mailbox”. The mailbox is then replaced within the actor reference with a system mailbox, redirecting all new messages “into the drain”."

This looks like there were a difference between the dead letter mailbox and "into the drain", but according to @rkuhn there is none. Therefore this part of the documentation should be rewritten correctly and more precisely.