This is a simple spam blocking technique that is often used. Users create a black list of from
addresses that should be prevented from entering the network and reaching the user’s inbox.
There are a few disadvantages with using this technique. Because spammers can create many
false from email addresses, it is difficult to maintain a black list that is always updated with the
correct emails to block. Also, some spammers do not even use a from address so a black list would
not be able to catch these cases. Even a rule to block emails without a from address would not be
sufficient because some legitimate emails, such as newsletters to which a user may subscribe,
may also not include a from address. Black list sender email addresses is effective in temporarily
blocking a small amount of spam but ineffective as an overall anti-spam solution.
As an alternative to black lists, some users set up an email white list consisting of acceptable
email addresses or domains. In this case, users only accept email from users that are listed on
their white list, while all other email is blocked. This technique poses many challenges as well
since people want to be able to receive email from people that they might not have entered into
their white list.
Some techniques will attempt to automatically build the white list from email that you have sent
or from your address book. This makes creating the list easier. However, it does not solve the
problems associated when people who legitimately want to send you email have not previously
corresponded with you via email, have multiple email addresses, or have a new email address.