Knowledge is Power Part II - Hotmail Spam Filters

Hello again. In my last article, I discussed the mechanism of how AOL Spam Filters operate. Continuing on the subject on Spam Filters, in this issue I will focus on Microsoft’s Hotmail Filter and delve into the basic Anti-Spam architecture and how it affects your email campaigns.

Microsoft uses multi-level spam filters for all inbound emails. The first level is the blacklist and volume filter check -- where your IP and sender domain are checked against Microsoft’s internal and 3rd party blacklists. Both throttling and blocking of connecting Mail Transfer Agent (MTAs) also happens at this level.

The email then passes through Brightmail’s spam filter and Smart Screen (Microsoft’s proprietary content Spam filter) and then is checked for whitelisting of the IP and sender domain against safe-lists and Sender Score. After passing this phase, it is verified for authentication i.e. SenderID -- which confirms sender domain is approved to be sent via the sending IP address. The recipient filter then checks if the from-address has been setup in the Recipient’s address book. A score is produced which determines the placement of your mail (Inbox, Junk or Trash).

Hotmail uses mailing history as one of the main determinant for placement of mails into the inbox. New IPs with no mailing history has a harder time for inbox placement compared to an old IP with reputation. Mailers can achieve and build this reputation by sending smaller volume regularly (5-10K per day) rather than large volumes infrequently. The consistency in the mailing has proved to have better performance on campaigns due to higher deliverability. Also keep in mind that, Hotmail does not use user’s address book as the determinant factor for mail placement unlike AOL -- as the address book is referenced late in the filtering hierarchy.

Over 30% of spam today is image spam, messages containing solely of one or multiple images, and Hotmail flags 90% of emails a day as spam. SPF, SenderID, and DomainKeys/DKIM records are very important to get your emails delivered along with Sender Score, Bonded Sender programs and Reputation of the sender.

Finally, the key to the inbox of Hotmail is heavily dependent on the reputation. The reputation metrics consists for three things:

-- Spam Complaints: Number of users reporting your campaign as spam
-- Unknown User Bournce Rates: Bad/Inactive email addresses
-- Spam Traps: Emails addresses acquired by harvesting or that was purchased

This is where emphasis on managing feedback loops, bounce management, and carefully selecting your data providers becomes critical and essential to building your reputation.