Which of the following statements about open e-mail relays is incorrect?

A.
An open e-mail relay is a server that forwards e-mail from domains other than the ones it serves.

B.
Open e-mail relays are a principal tool for distribution of spam.

C.
Using a blacklist of open e-mail relays provides a secure way for an e-mail administrator to identify open mail relays and fi lter spam.

D.
An open e-mail relay is widely considered a sign of bad system administration.

Explanation:
C: Although using blacklists as one indicator in spam filtering has its merits, it is risky to use them as an exclusive indicator. Generally, they are run by private organizations and individuals according to their own rules, they are able to change their policies on a whim, they can vanish overnight for any reason, and they can rarely be held accountable for the way they operate their lists. Page 827.