IFailFastChecker is what Rebus uses after having caught an exception, to decide whether it makes sense to retry delivery of the message.

By default, the fail fast checker will return true for exceptions that implement IFailFastException (which is an empty marker interface).

This means that you can make Rebus fail fast on certain exception types by adding that interface to your exceptions.

If you don't want to do that, or if you can't (e.g. if the exception type is not defined by you), then you can extend the behavior by adding a decorator. Check out Fail fast on certain exception types for an example on how to do this.