More detailed advice is given in the related documents referred to.
Unscrupulous marketers and others send millions of copies of a message to addresses all over the Internet, causing inconvenience and fruitless work for end users and system managers of all kinds. Spam is very unpopular and resources for its transmission cannot normally be obtained honestly. The bulk mail operators actively seek out mail systems anywhere in the Internet which they can use without authorisation to send their messages. They are insecure. Unwanted and undeliverable spam may swamp any open relay.