What is whitelisting and blacklisting and how does it work?From:
http://soe.uwm.edu/spamfaq.htm#SixWhitelists are lists of email addresses you always want to receive email from. Email that you "whitelist" does not get tested for spam although it will be scanned for viruses. You might, for example, whitelist a company you conduct business with if you want to receive their regular advertisements and not worry about the Barracuda quarantining messages from them. Tip: It isn't recommended to whitelist uwm.edu. While the Barracuda will give special consideration to email from UWM, spammers often use fake uwm.edu email addresses hoping to bypass spam filters. They won't bypass our filters by faking their address, but they will bypass yours if you whitelist uwm.edu.
Conversely, blacklists are lists of email addresses you never want to receive email from. Email that you "blacklist" is automatically blocked whether or not it is spam. On rare occasion, the Barracuda might not catch spam messages from a particular sender. In this case you could blacklist the email sender. Blacklisting is not always effective because not only do many spam senders fake their email address, they will often change their email address after they spam a bunch of people.
To add/delete email addresses from your personal whitelists and/or blacklists, visit the quarantine and click the Preferences Tab. Then click on the Whitelist/Blacklist sub-tab. Please visit question #1 if you need assistance accessing your quarantine. See the figure below for more information.
What is greylisting, and why does it stop spam?From :
http://spam.ucsd.edu/UCSD.asp#greylistingGreylisting is a technique that refuses to accept mail from mail senders that do not follow the proper procedure for delivering mail. When a message arrives, the sender is told to try again later. Proper procedures dictate that the sender wait a while, and try again.
This is how we separate legitimate mail senders from spammers. Spammers don't really care if a particular message gets through, so they usually do not try again later. Thus, we never accept the message, and it is not received by you.
Legitimate senders, however, will try again. When they do, we accept the message and put them in a list of legitimate mail senders. Once a mail sender has been shown to be legitimate, we remember them the next time they try to send a message, so that any future mail is accepted without delay. If many legitimate messages come through one mail server, all senders on that server are considered legitmate.