Summary
Microsoft Outlook may block emails for numerous reasons related to sender reputation, content, authentication, and recipient actions. These include a poor IP reputation (monitored via SNDS/JRMP), being listed on blocklists like Spamhaus (SBL), a high complaint rate, lack of proper email authentication (SPF, DKIM, DMARC), using content that triggers spam filters, and poor list hygiene. Recipient actions, such as manually blocking a sender, also cause blocking. Technical issues like un-warmed IP addresses, malformed headers, and blocked ports can contribute. Improving user engagement (via sending relevant emails) is also a significant factor. Addressing these issues can improve email deliverability to Outlook users.
Key findings
- Reputation Matters: A low IP or domain reputation significantly increases the likelihood of being blocked.
- Authentication is Key: Proper email authentication (SPF, DKIM, DMARC) is crucial for verifying sender identity and avoiding spam filters.
- Content Counts: Certain keywords, phrases, and formatting can trigger spam filters and lead to blocking.
- Lists Must Be Clean: Poor list hygiene (sending to inactive or unsubscribed users) negatively impacts sender reputation.
- Blacklists Are Detrimental: Being listed on blocklists directly results in blocked emails.
- User Engagement Matters: Microsoft weighs user engagement heavily when filtering emails; poor engagement will cause your email to go to junk or be blocked.
- Recipient Control: Recipients can manually block senders, preventing future email delivery.
Key considerations
- Monitor Reputation: Regularly check IP and domain reputation via tools like SNDS/JRMP and address any issues.
- Implement Authentication: Ensure SPF, DKIM, and DMARC are properly configured for all sending domains.
- Review Content: Avoid spammy keywords, excessive capitalization, and other triggers in email content.
- Maintain List Hygiene: Remove inactive subscribers and manage unsubscribes to keep email lists clean.
- Check Blocklists: Monitor blocklist status and take steps to be delisted if necessary.
- Improve Engagement: Send highly relevant emails to recipients that want them to improve user engagement.
- Warm Up New IPs: Gradually increase sending volume from new IPs to establish a positive reputation.
- Test Emails: Test emails across different clients (including Outlook) to ensure proper rendering.
- Address Microsoft Directly: Utilize the Microsoft ticket system (though initial responses may be canned), and be prepared to follow up.
What email marketers say
14 marketer opinions
Microsoft Outlook blocks emails for various reasons, primarily related to sender reputation, content, and authentication. Issues range from being listed on blacklists and having poor IP reputation to triggering spam filters with certain content or formatting. Technical issues like improper email authentication (SPF, DKIM, DMARC), blocked ports, and malformed headers can also lead to blocking. Engaging with Microsoft's SNDS/JRMP, managing complaint rates, practicing good list hygiene, and gradually warming up new IP addresses are crucial for maintaining deliverability. Furthermore, user engagement is a critical factor, and testing emails across different clients ensures proper rendering and reduces the likelihood of being flagged as spam.
Key opinions
- Reputation: Poor sender reputation (IP and domain) is a major cause of Outlook blocking emails.
- Authentication: Lack of proper email authentication (SPF, DKIM, DMARC) increases the likelihood of being blocked.
- Content: Certain words, phrases, and formatting in email content trigger spam filters.
- List Hygiene: Sending emails to unengaged users or maintaining poor email lists negatively impacts deliverability.
- Blacklists: Being listed on email blacklists causes Outlook to block emails.
- Engagement: Low user engagement (lack of opens, clicks) causes your email to go to junk or be blocked.
Key considerations
- Monitor Reputation: Regularly check your IP and domain reputation using tools like SNDS/JRMP and address any issues promptly.
- Implement Authentication: Ensure SPF, DKIM, and DMARC are properly configured to authenticate your emails.
- Review Content: Avoid using spammy keywords, excessive capitalization, and other elements that trigger spam filters.
- Clean Your List: Remove inactive subscribers and those who have unsubscribed to maintain a healthy email list.
- Request Delisting: If blacklisted, follow the delisting process outlined by the specific blacklist.
- Monitor Engagement: Focus on sending relevant emails to recipients that want them to improve the chance of opens, clicks and improve the overall user experience.
- Warm Up IP: Gradually increase sending volume from new IP addresses to build a positive reputation.
- Test Emails: Test emails across different clients, including Outlook, to ensure proper rendering and avoid spam triggers.
Marketer view
Email marketer from Reddit explains that certain words, phrases, or formatting choices in your email content can trigger spam filters. Avoid using excessive exclamation points, all caps, or spammy keywords like "free," "guarantee," or "urgent."
7 Oct 2021 - Reddit
Marketer view
Email marketer from StackOverflow explains that large senders need to set up feedback loops with ISPs like Microsoft. FBLs provide data about which users are marking your emails as spam, so you can remove those users from your list.
18 Feb 2023 - StackOverflow
What the experts say
4 expert opinions
Microsoft Outlook may block emails due to being listed on the Spamhaus Block List (SBL), a high complaint rate from recipients marking emails as spam, and a lack of email authentication (SPF, DKIM, DMARC). Microsoft also heavily considers user engagement, so poor engagement may lead to emails being sent to junk or blocked.
Key opinions
- Spamhaus Block List: Being listed on the Spamhaus Block List (SBL) can lead to blocked emails.
- High Complaint Rate: A high complaint rate from recipients marking emails as spam damages sender reputation and causes blocking.
- Lack of Authentication: Missing SPF, DKIM, and DMARC email authentication can result in emails being flagged as spam or blocked.
- Poor Engagement: Poor user engagement will cause your email to go to junk or be blocked.
Key considerations
- Check Spamhaus: Check the Spamhaus website to see if your IP or domain is listed and follow the delisting process if necessary.
- Monitor Complaint Rate: Monitor your complaint rates and improve list hygiene and content relevance to reduce complaints.
- Implement Authentication: Implement SPF, DKIM, and DMARC to authenticate your emails.
- Improve Engagement: Send highly relevant emails to recipients that want them to improve user engagement.
Expert view
Expert from Spam Resource states that a high complaint rate (recipients marking your emails as spam) can severely damage your sender reputation and cause Microsoft Outlook to block your emails. Monitor your complaint rates closely and take steps to reduce them, such as improving list hygiene and sending more relevant content.
8 Jul 2023 - Spam Resource
Expert view
Expert from Word to the Wise shares that Microsoft heavily weighs user engagement when filtering email, so poor engagement will cause your email to go to junk or be blocked. You should focus on sending highly relevant emails to recipients that want them.
28 Jan 2023 - Word to the Wise
What the documentation says
4 technical articles
Microsoft Outlook blocks emails if the recipient has blocked the sender. To ensure delivery, the sender can be added to the Safe Senders list. Poor IP reputation can also cause blocking, which can be monitored using SNDS. A common cause of delivery failure is a mistyped recipient email address.
Key findings
- Blocked Sender: Recipients blocking senders prevents email delivery.
- Safe Senders List: Adding senders to the Safe Senders list ensures email delivery.
- Poor IP Reputation: Poor IP reputation can cause emails to be blocked.
- Mistyped Addresses: Mistyped recipient email addresses lead to delivery failures.
Key considerations
- Unblock Sender: Recipients need to unblock senders in their Outlook settings.
- Request Safe Sender Addition: Request recipients add you to their Safe Senders list.
- Monitor IP Reputation: Monitor IP reputation using Microsoft SNDS.
- Verify Addresses: Verify recipient email addresses to avoid typos.
Technical article
Documentation from Microsoft Support explains that if a recipient has blocked a sender, emails from that sender will be blocked. The recipient needs to unblock the sender in their Outlook settings.
29 Nov 2022 - Microsoft Support
Technical article
Documentation from Microsoft SNDS explains that a poor IP address reputation can cause emails to be blocked. Monitor your IP reputation using SNDS (Smart Network Data Services) and ensure your sending practices are compliant with Microsoft's guidelines.
7 Jun 2023 - Microsoft SNDS
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