Messages of the _failure method family inform the sender of a problem on the recipient's side. A failure is the inability to accomplish a task (a request or other) on the side of the source issuing the _failure.
A classic example would be _failure_filter_strangers indicating that your message cannot be delivered to the recipient as she has denied reception of messages from strangers.
Standard derivate families of _failure are:
- _failure_deliver – could not deliver to destination.
- _failure_necessary – something needs to be provided on recipient's side.
- _failure_redirect – could not accomplish request because something has moved elsewhere.
- _failure_unavailable – something is not available although it should be.
- _failure_unsupported – something is not implemented although it should be.