Spam is something that is not likely to disappear anytime soon, lets face it, it's a way of life. To try and combat the problem some ISPs and users alike have opted to employ spam filtering software. Most of this software makes use of some sort of blacklist and/or whitelist, but what are they and how do they work?
When a mail server sending a message connects to the receiving server, the receiving server notes the sending server's IP address before it accepts any e-mail.
After identifying the sending server's IP address the receiving server can query any blacklists or whitelists currently available to determine if that IP address is on the list.
With the information gathered from the list, the receiving system can choose to accept or reject the e-mail and terminating the connection.
E-mail coming from blacklisted IP addresses tend to get rejected while e-mail from whitelisted IP addresses usually get delivered.
As more and more blacklists emerge to fight spam they tend to be more radical than the last. As a result blacklists have started to lose their impact. ISPs are finding blacklists, especially the less responsibly maintained ones, can lead to false positives and relying on them can lead to failure to deliver e-mail their users actually wanted.
As a result, some ISPs no longer take a blacklist listing as conclusive evidence that an e-mail marketer is guilty as charged.
At the same time, an alternative for e-mail marketers is developing... The whitelist... Permission-based e-mail distribution.
Getting an IP address on a whitelist is essentially telling the receiving server you should accept e-mail from this IP address. Most of the time that's what happens.
To get on a whitelists, the e-mail marketer must either contractually obligate itself to send only e-mail that meets the whitelist's requirements or put up a financial guarantee as to the cleanliness of its e-mail lists.
Some notes regarding both blacklists and whitelists: Few large ISPs use the lists as the be-all end-all. If they do use the lists, it's in an advisory capacity, in conjunction with other available information. One exception to this rule is Microsoft. The company recently announced it will start allowing senders listed on the Bonded Sender whitelist to go directly to Hotmail and MSN inboxes. A few ISPs including AOL, Yahoo and Earthlink, and spam filtering software maintain their own versions whitelists.
In an effort to make sure you mailings get though, it is important to remind new customers if they are using any sort of spam filtering software, they must add the address or domain of your e-mail to their whitelist. Otherwise there is a good chance your mail will never make it to their inbox.