Summary
Emails suddenly landing in Outlook and Hotmail spam folders can be attributed to a combination of factors. These include: a decline in sender reputation (which you can monitor using tools like SNDS), a sudden increase in spam complaints, changes in engagement rates, content triggering spam filters (including specific words/phrases and poor formatting), and inadequate or missing email authentication (SPF, DKIM, DMARC). Policy changes implemented by Microsoft can also play a significant role. It's crucial to maintain a clean and engaged email list, regularly check sender reputation, adhere to best practices for email content, and properly authenticate your emails to prevent deliverability issues.
Key findings
- Sender Reputation: A sudden drop in sender reputation is a primary cause for emails going to spam. Use tools like SNDS to monitor it.
- Engagement and Complaints: Outlook/Hotmail evaluates deliverability based on engagement rates and spam complaints. Low engagement and high complaint rates negatively impact deliverability.
- Email Authentication: Lack of proper email authentication (SPF, DKIM, DMARC) makes it easier for emails to be flagged as spam.
- Content Triggers: Using spam trigger words, poor HTML coding, broken links, or excessively large images in email content can trigger spam filters.
- List Hygiene: Sending to unengaged contacts and not maintaining a clean email list can negatively impact sender reputation and lead to spam filtering.
- Policy Changes: Microsoft's policy changes in spam filtering can cause sudden shifts in deliverability.
- IP Blocklists: Sender's IP address being on a blocklist can cause emails to be marked as spam.
Key considerations
- Monitor Reputation Tools: Regularly check your sender reputation using Microsoft's SNDS tool and other sender score websites.
- Implement Authentication Protocols: Ensure that SPF, DKIM, and DMARC records are properly configured for your sending domain.
- Avoid Spam Triggers: Review your email content and avoid using spam trigger words or phrases. Use a spam checker before sending.
- Clean and Segment Email Lists: Regularly clean your email list by removing inactive subscribers and segment your audience for more relevant content.
- Content Analysis: Check your email content for potential triggers and that it is not misleading to the recipient
- Engage with SNDS: If issues persist, register with Microsoft SNDS and engage with their support team to potentially get unblocked.
- Review Volume changes: Be conscious of any change in volume and avoid sudden large volume campaigns.
- Analyse Headers: Analyse the headers to identify the cause, such as reputation or SmartScreen issues, using the message analyzer tool.
What email marketers say
14 marketer opinions
Emails suddenly landing in Outlook and Hotmail spam folders can stem from various factors, including changes in sender reputation, engagement rates, spam complaints, content triggers, and email authentication. Experts recommend monitoring sender reputation through tools like SNDS, ensuring proper authentication with SPF, DKIM, and DMARC, avoiding spam trigger words and poor email formatting, and maintaining a clean and engaged email list. Policy changes at mailbox providers can also lead to sudden shifts in spam filtering.
Key opinions
- Reputation Impact: A sudden drop in sender reputation with Microsoft can cause emails to be filtered as spam. Monitor your reputation using tools like SNDS.
- Engagement Matters: Outlook/Hotmail measures deliverability based on engagement rates and spam complaints. Low engagement can negatively impact deliverability.
- Authentication is Key: Proper email authentication (SPF, DKIM, DMARC) is crucial for avoiding spam filters. Implement these records to improve sender verification.
- Content Triggers: Certain words, phrases, and poor email formatting can trigger spam filters. Avoid spam trigger words and ensure clean HTML coding.
- Policy Changes: Mailbox providers like Microsoft can change their spam filtering policies, leading to sudden deliverability issues.
- List Hygiene: Maintaining a clean and engaged email list is essential. Remove inactive subscribers and avoid sending to purchased lists.
Key considerations
- Monitor SNDS: Regularly check Microsoft's SNDS tool to monitor your sender reputation and identify potential issues.
- Review Authentication: Verify that your SPF, DKIM, and DMARC records are correctly configured and up-to-date.
- Evaluate Content: Review your email content for spam trigger words and poor formatting. Use a spam checker before sending.
- Segment Audience: Segment your audience and send relevant content to improve engagement rates.
- Clean List Regularly: Remove inactive subscribers and avoid sending to unengaged contacts to maintain a healthy email list.
- Investigate Header Analysis: If the cause is unknown, Analyze the headers to identify the cause, such as reputation or SmartScreen issues, using the message analyzer tool.
Marketer view
Email marketer from Reddit explains that sudden spam filtering can occur due to changes in email content or sending practices triggering spam filters, even if previous campaigns were unaffected. They recommend checking for recent changes and reviewing content for potential triggers.
3 Jan 2023 - Reddit
Marketer view
Email marketer from HubSpot explains that common reasons for emails going to spam include poor sender reputation, low engagement rates, spam trigger words, lack of email authentication, and sending to purchased lists. They suggest focusing on building a clean, engaged email list and following best practices for email content and sending.
6 Mar 2025 - HubSpot
What the experts say
2 expert opinions
Sudden deliverability issues with Outlook and Hotmail often stem from policy changes implemented by Microsoft, focusing on stricter authentication, engagement, and content rules. Maintaining a positive sender reputation is crucial, as a sudden drop can lead to emails being flagged as spam. Tools like Microsoft's SNDS are essential for monitoring sender reputation and identifying potential problems.
Key opinions
- Policy Changes: Microsoft frequently updates its spam filtering policies, impacting email deliverability.
- Reputation Matters: A drop in sender reputation directly correlates with increased spam filtering by Outlook/Hotmail.
- SNDS Tool: Microsoft's SNDS tool is crucial for monitoring sender reputation with Outlook/Hotmail.
Key considerations
- Stay Informed: Monitor Microsoft's documentation and tools for policy changes impacting deliverability.
- Monitor Reputation: Regularly check your sender reputation using SNDS to identify and address any issues.
- Review Authentication: Ensure your email authentication (SPF, DKIM, DMARC) is properly configured to meet Microsoft's requirements.
- Focus on Engagement: Improve email engagement to demonstrate positive sender behavior to Microsoft.
Expert view
Expert from SpamResource explains the importance of maintaining your sender reputation. If your reputation has dropped suddenly with Outlook/Hotmail (Microsoft), they are more likely to send your mail to the spam folder. You can check this via their SNDS tool.
16 May 2024 - SpamResource
Expert view
Expert from Word to the Wise explains that sudden shifts in spam filtering often result from policy changes at mailbox providers like Microsoft. Understanding specific changes requires digging into their documentation and tools, but generally involve tightening rules around authentication, engagement, and content.
16 Jan 2022 - Word to the Wise
What the documentation says
5 technical articles
Emails suddenly being marked as spam in Outlook and Hotmail can be attributed to several factors outlined in documentation from Microsoft, SparkPost, Google, RFC and DKIM. These include the sender's IP on a blocklist, email content triggering spam filters, recipients marking emails as junk, changes in sending volume, and infrastructure issues. Maintaining a positive sender reputation, proper email authentication (SPF and DKIM), monitoring bounce rates, feedback loops, spam rates, and following best practices for email content are crucial for preventing deliverability problems.
Key findings
- Blocklist Impact: A sender's IP address being on a blocklist can cause emails to be marked as junk.
- Content Triggers: Email content can trigger spam filters, leading to deliverability issues.
- User Feedback: Recipients marking emails as junk contributes to emails being filtered as spam.
- Volume Changes: Changes in sending volume can trigger temporary deliverability issues.
- Infrastructure Problems: Infrastructure problems can also affect email deliverability.
- SPF Prevents Forgery: SPF records prevent sender address forgery, improving deliverability.
- DKIM Adds Signature: DKIM adds a digital signature, verifying the sender and message integrity.
Key considerations
- Check Reputation: Regularly check sender reputation to identify and address potential issues.
- Monitor Metrics: Monitor bounce rates, feedback loops, and spam rates to identify root causes.
- Follow Best Practices: Follow best practices for email content to avoid triggering spam filters.
- Implement Authentication: Implement and maintain SPF and DKIM records for proper email authentication.
- Use Postmaster Tools: Leverage tools like Google Postmaster Tools to diagnose and resolve deliverability issues.
Technical article
Documentation from RFC explains that SPF records prevent sender address forgery. By publishing SPF records, domain owners can specify which mail servers are authorized to send email on their behalf, helping to prevent spammers from using their domain to send unsolicited messages.
4 Jul 2021 - RFC
Technical article
Documentation from DKIM explains that DKIM adds a digital signature to email messages, allowing recipients to verify that the message was sent by the authorized domain owner and has not been tampered with during transit. This helps improve email authentication and deliverability.
26 Jun 2023 - DKIM.org
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