Summary
Emails are crucial for marketing, but seeing them blocked by recipients can be incredibly frustrating. This issue, often referred to as email blockage or blacklisting, means your messages aren't reaching their intended inboxes, severely impacting campaign performance. Understanding why this happens is the first step toward resolution.
Key findings
- Internal IT policies: Corporate recipients often have stringent internal IT rules that block or quarantine emails, even if they appear delivered on the sender's side. Your IT team or the recipient's can investigate this.
- Authentication failures: Improperly configured email authentication protocols like DMARC, SPF, and DKIM can lead to emails being blocked or marked as spam by receiving mail servers, especially with stricter policies from major ISPs.
- Sender reputation: A low sender reputation, often caused by high spam complaints, sending to invalid addresses, or being on an email blacklist, can result in your emails being rejected outright or sent to the spam folder. Knowing how to determine if emails go to spam is essential.
- Content and engagement issues: Spammy content, irrelevant emails, or low engagement (poor open and click-through rates) can signal to ISPs that your emails are unwanted, leading to blocking.
- Blacklisting: Your sending IP address or domain might be listed on a public or private blacklist (also called a blocklist), causing receiving servers to automatically reject your emails. Understanding what being blacklisted means is important for diagnosis.
Key considerations
- Proactive monitoring: Regularly monitor your email deliverability and sender reputation. Tools and postmaster insights can highlight issues before they become critical.
- Communication with IT: If you suspect internal blocking, communicate with the recipient's IT team or your own to investigate logs and potentially whitelist specific sending IPs or domains.
- Authentication integrity: Ensure your SPF, DKIM, and DMARC records are correctly set up and aligned. Misconfigurations are a common cause of blocked emails.
- List hygiene: Maintain a clean email list by regularly removing invalid or inactive addresses to avoid bounces and spam trap hits. Sending to the right audience is paramount.
- Content optimization: Avoid spam trigger words, excessive links, and poor formatting. Focus on providing value to recipients to encourage engagement and reduce complaints.
What email marketers say
Email marketers frequently encounter situations where their campaigns, despite appearing successful on their end, fail to reach recipients. These issues often stem from a combination of technical configurations, sender reputation challenges, and the complexities of recipient server filtering. Marketers highlight the importance of understanding bounce reasons and closely collaborating with IT teams or ESPs to diagnose and fix deliverability problems.
Key opinions
- Migration challenges: Migrating email services, especially during critical periods like major sales events, can lead to unexpected deliverability issues, including hard bounces and suppression.
- Hidden blocks: Emails might appear 'delivered' by the ESP but are actually blocked or quarantined at the recipient's mail server or by their internal IT rules, making diagnosis difficult.
- Suppression lists: Unforeseen hard bounces can automatically place recipient addresses on suppression lists, preventing future email delivery until manually removed.
- Content and engagement: Sending too many irrelevant emails to the wrong people or having low open rates can trigger blockages and spam filters.
- IP address issues: Shared IP addresses with poor reputations can lead to email marketing being blocked by major ISPs like Gmail.
Key considerations
- Strategic migration: Avoid critical periods for major email system migrations to minimize unexpected deliverability issues and disruptions.
- Review bounce reasons: Thoroughly investigate the reasons for hard bounces and suppression to identify underlying problems, such as MailBlockKnownSpammer errors.
- Engagement optimization: Focus on sending relevant and engaging content to the right audience to improve open rates and reduce spam complaints.
- Content quality: Ensure your email content is not perceived as spammy, which can lead to emails being caught in spam filters or blocked altogether.
- Proactive outreach: If encountering issues, consider testing emails to different providers (e.g., Gmail) to isolate the problem. If issues persist with a specific provider like Outlook, refer to guides on Outlook deliverability issues.
Marketer view
Email marketer from Email Geeks explains that after migrating from Gmail to Outlook, their marketing emails, specifically test emails in Klaviyo, were not delivered to Outlook, nor were they found in spam, junk, or focused folders. This unexpected blockage occurred despite the platform indicating successful sending.
Marketer view
Email marketer from Nutshell notes that a significant number of sales emails get caught in spam filters due to factors like emailing too many people at once or having consistently low open rates from past campaigns. These behaviors severely impact deliverability.
What the experts say
Email experts stress that diagnosing email blockages requires a systematic approach, often involving collaboration with the recipient's IT department. They emphasize that while DMARC policies can play a role, corporate-level filtering and the configuration of email sending infrastructure are usually the primary culprits. Whitelisting should be approached cautiously and precisely.
Key opinions
- IT team involvement: There is a high probability that emails are being blocked by rules implemented by the recipient's IT team. They are the first point of contact for investigation.
- Investigation is key: Understanding the precise reason for a block requires an investigation, after which a relevant rule or configuration adjustment can be put in place. This includes checking if your emails are going to spam.
- Whitelisting precision: Whitelisting an entire Email Service Provider (ESP) is generally not recommended. Instead, specific IP addresses from the ESP should be whitelisted if necessary.
- DMARC policy impact: While DMARC policies are important, a DMARC reject policy problem might not be the primary cause of immediate blocking if other corporate filters are in place.
- Comprehensive analysis: The path of an email from the ESP to the local inbox (ESP -> internet -> Microsoft Cloud/Exchange -> local Outlook) must be analyzed to pinpoint where the message was stopped. Tools like Mailgun's deliverability insights can assist.
Key considerations
- Collaboration with recipients: Engage with the recipient's IT team to get logs or insights into why specific emails are being blocked at their gateway or mail filter level.
- Targeted whitelisting: If whitelisting is required, request your ESP to provide the specific IP addresses or domain names that need to be approved, rather than broad whitelisting.
- Check DMARC configuration: While not always the direct cause, ensure your DMARC records are correctly configured and aligned to prevent authentication failures that can lead to blocking. Learn about DMARC verification failed errors.
- Monitoring deliverability: Continuously monitor your deliverability rates to various ISPs and domains. This helps identify trends and specific blocking patterns.
- Understand blocklist mechanisms: Familiarize yourself with how email blacklists work to better diagnose and remediate issues when your IP or domain gets listed.
Expert view
Email expert from Email Geeks suggests that corporate IT teams or their service providers are often responsible for blocking emails through internal rules. They emphasize that such teams typically need to whitelist the sender's ESP or specific IPs to resolve the issue.
Expert view
Email expert from Word To The Wise observes that poor sending practices, such as sending to unengaged users or using purchased lists, can quickly damage a sender's reputation, leading to blocklisting by ISPs and email service providers.
What the documentation says
Official documentation from major email service providers and security entities consistently highlights the importance of sender authentication and reputation for email deliverability. They often provide tools and guidelines for diagnosing issues when emails are blocked or quarantined. Compliance with best practices, including robust authentication and responsible sending, is repeatedly emphasized.
Key findings
- Advanced hunting: Platforms like Microsoft 365 Defender offer advanced hunting features that allow mail administrators to locate and understand why emails are being blocked or quarantined after being accepted by the mail server.
- Authentication compliance: Email providers like Gmail and Yahoo have new, stricter requirements for bulk senders, mandating proper SPF, DKIM, and DMARC authentication to ensure emails are not blocked.
- Sender reputation metrics: ISPs use various signals to assess sender reputation, including spam complaints, bounce rates, and user engagement, which directly influence whether emails are blocked or inboxed.
- Consent and relevance: Documentation often stresses the importance of explicit recipient permission and sending relevant content to avoid being marked as spam, a key factor in preventing blockages.
- Monitoring tools: Tools such as Google Postmaster Tools provide insights into domain reputation and spam rates, enabling senders to proactively identify and address deliverability issues. Consult the ultimate guide to Google Postmaster Tools V2.
Key considerations
- Utilize diagnostic tools: Leverage built-in security features of email platforms (e.g., Microsoft 365 Defender) to trace email delivery paths and pinpoint reasons for quarantining or blocking.
- Adhere to sender requirements: Stay updated and comply with all sender requirements published by major ISPs like Gmail and Yahoo, especially concerning authentication standards and complaint thresholds.
- Manage sender reputation: Proactively manage your domain and IP reputation by sending high-quality, relevant emails and minimizing spam complaints and bounces.
- Implement DMARC, SPF, and DKIM: Ensure these authentication methods are correctly configured and aligned to verify sender identity and prevent spoofing, which helps prevent blockages. A simple guide to DMARC, SPF, and DKIM can assist.
- List hygiene best practices: Routinely clean your mailing lists to remove inactive users and addresses that generate hard bounces or are known spam traps.
Technical article
Official documentation from Microsoft suggests that mail administrators can use features like Microsoft 365 Defender's advanced hunting to locate emails that may have been blocked at the mail filter level, even after the mail server initially accepted them. This tool helps in identifying the specific reasons for quarantine.
Technical article
Documentation from Mailchimp emphasizes that emails are sometimes marked as spam because the sender lacks explicit permission to email that recipient. Obtaining express permission is crucial to avoid spam filters and subsequent blockages.
Related resources
15 resources
Related pages
How to determine if marketing emails are going to spam?
Why are my emails having deliverability issues with Microsoft Outlook and Hotmail?
Why are transactional emails bouncing with 'MailBlockKnownSpammer' and how to resolve it?
Why Your Emails Are Going to Spam in 2024 and How to Fix It
What does it mean when your email is blacklisted?
Why your emails are getting a 'DMARC verification failed' error
How email blacklists actually work: a simple guide
Email Deliverability Issues: Getting Your Messages to the Inbox in 2025
Spam traps: what they are and how they work
A simple guide to DMARC, SPF, and DKIM
