Just because it cost me a few hours, I'd like to share the following insight, may it be interesting for someone: If we want to handle dead letters by ourselves, the akka.net documentation (https://getakka.net/articles/utilities/event-bus.html) says we should use
"system.EventStream.Subscribe(deadletterWatchActorRef, typeof(DeadLetter));"
This is only partly true - yes, this catches those messages which are strictly "dead letters", those whose recipient is currently dead. But it does not catch the unhandled messages and the dropped ones. If you want that (and I'm pretty sure that 99% of the use cases don't want to finely differentiate between unprocessed, dropped and really dead letters), you have to use "AllDeadLetters" instead of "DeadLetters", so the proper call is
system.EventStream.Subscribe(deadletterWatchActorRef, typeof(AllDeadLetters));