Why is it important to adopt a sender authentication protocol?Adoption of sender authentication protocols is not merely recognized as a best practice; it may have an impact on email delivery. For example, Microsoft announced plans to begin inserting visual warnings in the MSN Hotmail user interface on messages where sender authentication cannot be verified. The new warning was to be made available in 20 languages supporting over 200 million MSN Hotmail users worldwide. Implementation of the new interface began in June 2005.
Microsoft is deploying its SenderID checks in three phases: In the first phase, which began in June 2005, implementation applied only to email senders that publish SPF records which fail the authentication check due to a mis-match between the IP address and domain or due to an incorrectly published SPF record. Email failing the SenderID check may be placed in the inbox with a warning icon as illustrated below: 
In the second phase, senders that publish SPF records which fail the authentication check due to a mis-match between the IP address and domain or due to an incorrectly published SPF record may be placed in the junk mail folder or rejected altogether based on the sender's reputation and other anti-spam heuristics. Messages that fail SenderID checks and email from domains without SPF records will face further scrutiny. Also, email messages originating from invalid domains will be sent to the junk mail folder. Microsoft will be testing and adjusting its filtering policies during this second phase.
Finally, in the third phase, Microsoft will begin routing all email either failing the authentication check or lacking a published SPF or SenderID record to the junk folder.
In 2006, Yahoo! will begin requiring senders that wish to establish a feedback loop to utilize the DomainKeys protocol. Gmail is checking and validating DomainKeys records for incoming email, but is not currently taking any action as a result of its checks.
Habeas has been promoting the use of sender authentication for some time, but Microsoft's and Yahoo's announcements makes adoption of sender authentication more urgent in order to ensure continued delivery of email. No cost is associated with creating an sender authentication records, and correct sender authentication records can only improve delivery rates, not damage it.
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