n. When a piece of email or netnews disappears mysteriously between its origin and destination sites (that is, without returning a bounce message) it is commonly said to have 'fallen into a black hole'. "I think there's a black hole at foovax!" conveys suspicion that site foovax has been dropping a lot of stuff on the floor lately (see drop on the floor). The implied metaphor of email as interstellar travel is interesting in itself.
Compare bit bucket.