In the last few years new Anti-Spam Technologies evolved in attempt to eliminate or even reduce the amount of Spam. Vendors and Open Source Software Solutions were based on one or more combinations of the following technologies:

1. Keyword Filtering
2. Spam IP Database Block Lists
3. Black List Addresses
4. White List Addresses
5. Challenge-Response
6. Bayesian Filtering

In retrospect, these technologies failed to handle the source of the problem, authenticating the sender.

There are three competing technologies that are based on Sender Authentication.

• Yahoo!^® DomainKeys
• Microsoft^® Sender ID
• Simplicato Address Authentication

Obviously, we have the most knowledge about Email Address Authentication. Our understanding of Yahoo!^® DomainKeys and Microsoft^® SenderID is limited by what was available to us on the Internet.

Feature	Yahoo!® DomainKeys	Microsoft® Sender ID / SPF	Simplicato Address Authentication
Domain Level Key	Yes	Yes	Yes
Email Address Level Key	No	No	Yes
Handling Forward	No	No	Yes
Handle Email Spoofing within Domain	No	No	Yes
Handling Multiple Email Accounts/Domains with one set of SMTP Servers	No	No	Phase I - No, Phase II - Yes
Require Public / Private Key	Yes	No	No
Prevent Spam within the domain	No	No	Yes
Require additional CPU Power	Yes	No	No
Handle Third Party Sending	No	No	Phase I - No, Phase II - Yes
Simple to implement	No	Yes	Yes
Patent	Yes	Yes	Pending
License Used	Yahoo!®	Microsoft®	Open Source License
DNS Changes	Yes. Public Key Record	Yes. List of SMTP Server	Yes. List of Authentication Servers
Handle Multiple hops	No	No	Yes
Transparent to Users	Yes	Yes	Yes
Prevent Phishing within the Domain	No	No	Yes
Support Multiple Domains in SMTP Servers	Yes	No	Yes

Notes: Public / Private Key Processing requires additional CPU Power and may affect existing SMTP Servers.