Suppose mail is accepted by the system, and then for some reason cannot be delivered, so it bounces back to the sender. This is called a delayed bounce, i.e., a bounce that occurs some time after incoming mail has been accepted.
If the sender address was forged, the bounce goes to some innocent person who will consider it a type of spam. This is called backscatter. Almost all spam comes with a forged sender address, so either spam should be rejected and never accepted or, once accepted, should not be bounced back.
Backscatter will cause our servers to be added to various black lists, and will then cause outgoing mail from our servers to be treated as spam and possibly rejected by receiving sites.
For some ways of dealing with backscatter, see: