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Microsoft (Outlook.com and Hotmail) spam filter factors

Getting good inbox placement at Microsoft involves understanding the way Microsoft’s spam filter works and implementing best practices Microsoft considers to be important. Following Microsoft's best practice guidelines builds trust and helps you to maintain a good sending reputation, which helps to ensure your email reaches the inbox.

To minimize the impact junk email has on its users’ email experience, Microsoft uses a combination of filtering technologies, reputation measures, whitelists, blacklists and other factors in its spam filtering formula. These include:

Sending reputation

Deliverability to Microsoft is based on a sender's reputation. Your sending reputation is a measure of how likely your email is spam or not spam. Common metrics that make up your sending reputation (among many others) include subscriber complaints, sending email to unknown users, sending email to spam traps, and how much your subscribers interact and engage with your email. Sending reputation at Microsoft is always the responsibility of the sender.

Sender Reputation Data (SRD)

Filtering is heavily influenced by Microsoft’s Sender Reputation Data (SRD) program. Users that are invited to be members of the SRD panel can vote on emails they have received as Not Spam or Spam. A Spam vote from an SRD panel member carries a lot of weight in determining if a message is spam. Microsoft regularly tunes its SRD voting sample size to ensure the panel is representative for all senders.

Complaints

Standard complaints from Microsoft users are recorded when they click on the This is Spam button. Microsoft counts each complaint as unique. It does not remove duplicates for a specific sender if the complaints occur in a short period of time. For example, if 100 messages from a sender are marked as spam from a user during one instance, 100 complaints are recorded.

Engagement

Microsoft considers engagement when determining whether to place an email in the inbox. Recipients that open your email, reply to it and move your email out of the junk folder help Microsoft have more trust that your email is not spam. Recipients that do not open your email or delete it without reading lead Microsoft to believe your email is more likely to be spam.

Authentication

Microsoft uses Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC) to verify that messages came from the purported domain. They encourage all senders to use SPF and DKIM to protect their recipients from junk email and phishing scams. They also recommend publishing a DMARC record, which is used to reject or quarantine email sent from unauthorized senders.

SmartScreen®

The SmartScreen® content filter learns from known spam and phishing threats, user feedback, as well as from Microsoft users who have opted to be part of their junk email classification program. This data helps SmartScreen® identify legitimate and junk email and provides key metrics for sender reputation. 

Symantec Brightmail

Symantec's patented technology identifies and eliminates junk email before it reaches a Microsoft users' inbox. Symantec protects against unsolicited junk email by offering a dynamic technology that keeps pace with constantly evolving junk email. Microsoft does not disclose how it implements the filter in its systems.

Personal filters

Microsoft gives users the ability to set personal filter levels to further improve the delivery of email to their account. Users can easily add a sender or domain name to the Safe Senders and Domains List so that the email from that sender or domain is never treated as junk regardless of the content of the message. Conversely, users can enable an exclusive mode to accept only messages from the Contacts and Safe Senders List.

Whitelists

Return Path's Certification program is an industry leading accredited whitelist used by Microsoft to help identify best-in-class senders. Due to strict acceptance standards, Microsoft's spam filter gives preferential deliverability treatment to whitelist participants. 

Blacklists

Spamhaus is an industry leading 3rd party blacklist provider used by Microsoft to help identify potential spammers and other potential threats such as phishing and malware websites. Senders listed at Spamhaus see blocked messages at Microsoft as a blacklisted IP address may indicate a source of spam or other potential threat.

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