Summary
Emails suddenly going to spam in Outlook can be attributed to a confluence of factors. Changes in Microsoft's filtering algorithms, issues with sender reputation (including blacklisting and drops in IP score), and characteristics of email content (trigger words, excessive links) play significant roles. Low user engagement, poor list hygiene, sudden spikes in email volume, and even "list bomb" attacks can negatively impact deliverability. Furthermore, proper email authentication using SPF, DKIM, and DMARC is critical. Monitoring sender reputation through tools like SNDS, Google Postmaster Tools, and third-party blacklist checkers is also essential for identifying and resolving deliverability issues. Microsoft's technical difficulties can also contribute, as can problems with ticket resolution with them.
Key findings
- Outlook Filtering Updates: Microsoft frequently updates its spam filters, which can cause sudden shifts in deliverability.
- Sender Reputation: Poor IP and domain reputation (including blacklisting) are primary drivers of spam filtering.
- Content Triggers: Specific words, phrases, or excessive links can trigger spam filters.
- User Engagement: Low open rates, high deletion rates, and other low engagement signals hurt deliverability.
- Authentication Issues: Lack of proper SPF, DKIM, and DMARC configuration makes emails more susceptible to spam filtering.
- Volume and Reputation: Sending volume must align with reputation; sudden volume increases without established reputation trigger spam filters.
- List Hygiene: Poor list hygiene (sending to unengaged users) increases the risk of spam filtering.
- List Bombing: A surge of subscriptions via list-bombing can cause deliverability issues.
- MSFT Technical Issues: Microsoft sometimes experiences technical difficulties leading to spam filtering issues.
Key considerations
- Implement Authentication: Implement and maintain SPF, DKIM, and DMARC records to authenticate emails.
- Monitor Reputation: Regularly monitor IP and domain reputation using tools like Sender Score, Google Postmaster Tools, and SNDS.
- Check Blocklists: Use blacklist checkers to proactively identify and address blacklisting issues.
- Improve Engagement: Segment email lists and send relevant content to improve user engagement.
- Clean Email Lists: Remove unengaged subscribers from email lists to improve deliverability.
- Review Content: Carefully review email content for spam triggers.
- Manage Sending Volume: Ramp up sending volume gradually and maintain a consistent sending cadence appropriate for your sender reputation.
- Report Issues: Use tools like Junk Email Reporting add-in to report false positives and negatives to Microsoft.
- Review SNDS and Header Scores: Review SNDS and header scores when raising a ticket with Microsoft.
What email marketers say
10 marketer opinions
Emails suddenly going to spam in Outlook can be attributed to a variety of factors. These include changes in Outlook's filtering algorithms, blacklisting, sudden spikes in email volume, poor list hygiene, and low user engagement. Specific content within emails, such as certain words or excessive links, can also trigger spam filters. Maintaining a good IP address reputation, participating in feedback loops, and ensuring proper authentication (SPF, DKIM, DMARC) are crucial for preventing deliverability issues. It's also essential to monitor SNDS, header scores, and technical difficulties at Microsoft, as well as investigate potential 'list bomb' attacks.
Key opinions
- Filtering Changes: Outlook frequently updates its spam filtering algorithms, leading to sudden changes in deliverability.
- Reputation Impact: Poor IP and domain reputation can cause emails to be filtered as spam.
- Content Triggers: Specific words, phrases, or excessive links in email content can trigger spam filters.
- Engagement Matters: Low user engagement (low open rates, high deletion rates) negatively impacts deliverability.
- Authentication: Lack of proper authentication (SPF, DKIM, DMARC) can lead to deliverability issues.
- Volume Spikes: Sudden increases in email volume, particularly without an established sender reputation, can trigger spam filters.
- List Hygiene: Poor list hygiene (sending to inactive or unengaged users) increases the likelihood of emails landing in spam.
- Blacklisting: Being blacklisted by spam filtering organizations will cause emails to go to spam.
- Technical Difficulties: Microsoft sometimes experiences technical issues that can affect email deliverability.
- List Bombing: List bomb attacks can cause increased spam filtering.
Key considerations
- Monitor Reputation: Regularly check your IP and domain reputation using tools like Sender Score and Postmaster Tools.
- Authenticate Email: Ensure proper email authentication (SPF, DKIM, DMARC) is configured to verify your identity.
- Review Content: Carefully review email content for spam triggers and test emails with a tool like Mail-Tester.
- Improve Engagement: Segment your email list and target users with relevant content to improve engagement rates.
- Maintain List Hygiene: Regularly clean your email list by removing inactive or unengaged users.
- Participate in Feedback Loops: Set up feedback loops with Outlook to identify and address spam complaints.
- Monitor SNDS: Monitor SNDS to understand your IP's performance.
- Check Blocklists: Use a blocklist checker to ensure your IP isn't blacklisted.
Marketer view
Email marketer from Email Marketing Forum shares that being blacklisted, a sudden spike in email volume, or poor list hygiene can cause emails to suddenly land in Outlook's spam folder. They recommend using a blacklist checker and segmenting email lists.
28 May 2022 - Email Marketing Forum
Marketer view
Email marketer from MarketingProfs explains that certain words, phrases, or excessive links in email content can trigger spam filters. They suggest reviewing email content for spam triggers and testing emails with a tool like Mail-Tester.
25 Feb 2025 - MarketingProfs
What the experts say
3 expert opinions
Emails suddenly landing in Outlook's spam folder can be due to several factors highlighted by experts. Microsoft frequently adjusts its filtering logic, leading to unexpected deliverability changes based on user behavior or new spam tactics. It's crucial to monitor if your sending IPs have been blocklisted by common providers, using tools to check against multiple lists. Additionally, maintaining consistent sending volumes aligned with your reputation is essential, as sudden increases can trigger spam filters if your reputation isn't established.
Key opinions
- Filtering Logic: Microsoft's frequent adjustments to filtering logic impact deliverability.
- Blocklisting: Being blocklisted is a primary cause of emails going to spam.
- Volume & Reputation: Inconsistent sending volumes relative to reputation trigger spam filters.
Key considerations
- Monitor Changes: Stay updated on Microsoft's filtering logic and adapt email practices accordingly.
- Check Blocklists: Regularly monitor blocklists to ensure your sending IPs aren't listed.
- Maintain Consistency: Maintain sending volumes that align with your established sender reputation.
Expert view
Expert from Word to the Wise explains that Microsoft makes frequent adjustments to its filtering logic, which can sometimes lead to unexpected changes in deliverability. These shifts can be triggered by various factors, including changes in user behavior or new spam tactics.
3 Dec 2024 - Word to the Wise
Expert view
Expert from Word to the Wise highlights that senders should ensure their sending volumes are consistent with their reputation. A sudden, large increase in volume without an established reputation may trigger spam filters.
5 Aug 2021 - Word to the Wise
What the documentation says
5 technical articles
Emails suddenly being marked as spam in Outlook is influenced by several factors including sender reputation, email content, user complaints, and proper authentication. Microsoft recommends using the Junk Email Reporting add-in to address false positives. Implementing SPF records prevents spammers from forging 'From' addresses. DKIM allows organizations to take responsibility for messages, ensuring they haven't been altered in transit. DMARC builds on SPF and DKIM to protect against spam, phishing, and spoofing. Google Postmaster Tools can also assist in monitoring deliverability.
Key findings
- Spam Filtering Factors: Sender reputation, email content, user complaints, and authentication affect spam filtering.
- SPF Prevents Forgery: SPF records prevent spammers from forging 'From' addresses.
- DKIM Ensures Integrity: DKIM verifies that a message hasn't been altered in transit.
- DMARC Protects Against Abuse: DMARC protects against spam, phishing, and spoofing.
- Google Postmaster Tools: Google Postmaster Tools can be used to monitor deliverability.
Key considerations
- Monitor Reputation: Actively monitor and maintain a good sender reputation.
- Implement SPF: Implement SPF records to prevent email address forgery.
- Use DKIM: Use DKIM to ensure message integrity.
- Deploy DMARC: Deploy DMARC to enhance email security.
- Use Reporting Tools: Use reporting tools to address false positives and improve filtering.
Technical article
Documentation from DKIM explains that DomainKeys Identified Mail (DKIM) lets an organization take responsibility for a message by associating a domain name with it. This allows mail systems to verify that an incoming message from a domain was authorized by that domain's owner, and that the message has not been altered in transit.
16 Dec 2021 - DKIM
Technical article
Documentation from RFC explains that implementing SPF records prevents spammers from sending messages with forged 'From' addresses at your domain. When a mail server receives a message claiming to be from your domain, the server can query your DNS to determine if the source IP address is authorized to send messages from your domain.
20 Dec 2022 - RFC
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