A spam filter is a program of a mail rule setup to block emails based on certain criteria. For example, if you want to block emails about low mortgages a filter would be set to scan your emails for the word mortgage. Other parts of an email can be filtered as well, like the email address from which spam emails may come from. You could set a filter to block emails if the word mortgage is in any part of an email address like lowmortgages@ispam.com.

Spam Filters just don't do the job...
For the most part a spam filter will stop a number of spams but people who send spam are creative. They rotate the email address from which they send the spam and are always changing the wording of the spam they send to trick of fool spam filters. Spam filters will always let some spam through. One other drawback to spam filtering is that they can from time to time block an email that you should get. Maybe you have decided to buy a home and your banker or realtor has emailed you about raising mortgage rates. The spam filer for the word mortgage would block it and you would miss the email.

Mail Unknown DOES NOT rely on spam filtering technology. Mail Unknown does have the ability to block spam at a domain level. You have full control of the domains that are blocked or authorized to email you. The domain blocking feature of Mail Unknown should only be used when you are 100% sure that all email coming from the domain will be spam. Blocking a domain like HotMail.com or Yahoo.com is a very bad idea even though spam is sent from those domains. Mail Unknown will be able to block spam from those domains using it's Verify and Validate technology.