Summary
Emails being flagged as phishing or not delivering to Outlook can stem from a variety of technical and reputational issues. While recent changes by Microsoft might tighten filters, the root causes often involve misconfigured authentication, poor sender reputation, or malicious elements like compromised tracking pixels. Understanding Microsoft's aggressive spam and phishing detection mechanisms is crucial for maintaining good inbox placement. Many factors contribute to deliverability challenges, highlighting the complexity of email ecosystems and the importance of diligent monitoring.
Key findings
- Tracking pixel issues: A primary cause of emails being flagged as 'PHISH' in Outlook can be a suspicious or deprecated tracking pixel within the email content. Microsoft's filters are highly sensitive to such elements, especially if the domain associated with the pixel has a low reputation or has been compromised.
- Shared reputation risks: If you are using third-party services for tracking (e.g., ad networks or email service providers), their pixel or hosting domain's reputation directly impacts your email deliverability. A compromised or poorly managed third-party domain can cause your legitimate emails to be blocklisted.
- Microsoft's stringent filtering: Microsoft (Outlook, Office 365, Hotmail) employs robust spam and phishing detection. Even minor issues with content or sender reputation can lead to emails being sent to junk or blocked entirely. This is often reflected in metrics like Spam Confidence Level (SCL) and Bulk Complaint Level (BCL).
- Internal email practices: Practices like BCCing internal inboxes for record-keeping can sometimes lead to internal rate limiting or misinterpretation as bounces, negatively affecting overall sender reputation with Microsoft.
- Deprecated elements: Using deprecated or unused tracking pixels and other older elements can inadvertently trigger modern spam filters, even if they functioned fine historically.
Key considerations
- Thorough content audit: Regularly review all links, images, and tracking pixels within your emails. Ensure that any third-party domains associated with these elements have good standing and are not on any email blacklist or blocklist.
- Authentication standards: Properly configure and maintain your SPF, DKIM, and DMARC records. These authentication protocols are foundational for proving email legitimacy and avoiding phishing flags.
- Sender reputation management: Monitor your sender reputation continuously. Factors like complaint rates, bounce rates, and engagement metrics (opens, clicks) play a significant role. Tools for blocklist monitoring can help identify issues early.
- Review internal sending practices: Evaluate how internal emails or proofing processes are handled, especially if they involve BCCing or high volumes, as these might inadvertently affect your sending reputation with Microsoft.
- Stay updated on ISP policies: Email service providers like Microsoft regularly update their filtering algorithms. Staying informed about these changes (for example, via Microsoft's verification updates) is vital to adapt your sending strategy and prevent deliverability issues.
What email marketers say
Email marketers often face unexpected deliverability challenges, especially with major providers like Outlook. These issues can be frustrating and difficult to diagnose without specialized knowledge. Their experiences highlight how subtle elements, like tracking pixels or internal sending practices, can suddenly trigger strict spam or phishing filters, leading to significant disruptions in email campaigns and communication. The reliance on third-party vendors also introduces shared reputation risks, making it harder to control deliverability outcomes directly.
Key opinions
- Sudden deliverability drops: Many marketers report experiencing abrupt declines in Outlook deliverability, often attributing it to unannounced policy changes or filter updates by Microsoft.
- Tracking pixel sensitivity: Specific tracking pixels, even those used for a long time, can suddenly be flagged as 'PHISH' by Microsoft, indicating heightened scrutiny of embedded content.
- BCC impact: Using BCC for internal record-keeping or testing can inadvertently lead to rate limiting or negatively affect sender reputation with Microsoft.
- Deprecated technology: Emails containing elements from deprecated products (e.g., old tracking pixels) are prone to being flagged by modern email filters.
- Dependency on ESPs: Marketers often rely on their Email Service Providers (ESPs) like Responsys or clients for troubleshooting complex deliverability issues when in-house expertise is lacking.
Key considerations
- Content hygiene: Regularly audit email content for suspicious keywords, poor subject lines, and outdated tracking elements that could trigger spam filters. Remember that a simple tracking pixel can have a significant impact.
- Sender reputation awareness: Be aware that your sender reputation is dynamic and influenced by various factors, including bounce rates (like hard bounces) and engagement metrics. Shared IP/domain reputation, especially from ESPs, can also play a major role, as discussed in our guide on Outlook.com deliverability.
- Monitoring: Implement monitoring for email deliverability to quickly identify and respond to changes in inbox placement. This includes observing specific flags like 'PHISH' warnings.
- Engagement metrics: Low open rates and high unsubscribe rates are signals that can negatively impact your sender reputation, leading to more emails being flagged as spam.
- Test thoroughly: Before broad deployment, conduct thorough testing to various mailbox providers to ensure emails are not being flagged or blocked. This can help catch issues before they affect a large audience.
Marketer view
Email marketer from Email Geeks observed a sudden decline in Outlook deliverability around a specific date, preventing the receipt of internal proofs. This disruption suggested a potential, unannounced update from Microsoft that might be automatically excluding promotional emails or tightening filters. The intensity of the issue indicated a significant shift in filtering behavior.
Marketer view
Email marketer from Email Geeks discovered that a specific tracking pixel within their emails was getting flagged by Microsoft as 'PHISH.' Upon removing this pixel, test emails successfully went through. This indicated that Microsoft likely changed something in their filtering algorithms, as the pixel had been in use without issues for a long time.
What the experts say
Experts emphasize that email deliverability issues, particularly those leading to phishing flags, are rarely simple. They delve into the underlying technical configurations, shared infrastructure risks, and the nuances of how mailbox providers like Microsoft interpret sender behavior. Their insights highlight the critical role of proper authentication, continuous monitoring, and understanding the cascading effects of shared reputation in the complex email ecosystem.
Key opinions
- Specific phishing flags: When an email is flagged specifically as 'PHISH', it indicates a severe issue. This often points to the sender hosting a phishing site or being compromised.
- Shared domain reputation: Using third-party domains for tracking pixels or hosting means you share their sender reputation. If other users on that domain are compromised or engage in malicious activities, it can negatively impact your deliverability.
- Domain validation by filters: Spam filters actively validate domains. If your domain or an associated third-party domain is on an email blocklist (or blacklist) or has low trust, emails are likely to be flagged or blocked.
- Algorithmic changes: Sudden drops in deliverability to a specific ISP often suggest an algorithmic change or increased sensitivity to certain content or sending patterns by the mailbox provider.
- Authentication importance: Properly configured authentication (SPF, DKIM, DMARC) is essential for proving email legitimacy and preventing spoofing, which directly influences deliverability and avoids phishing warnings.
Key considerations
- Immediate investigation for 'PHISH' flags: Any 'PHISH' flag requires immediate investigation of the flagged domain (e.g., tracking pixel) to rule out hosting a phishing site or being part of a compromised network. This is a critical security and deliverability concern, as detailed in our article why emails trigger phishing warnings.
- Domain ownership and reputation: Verify if the domain associated with any tracking elements belongs to you or a third party. If it's a third party, understand that their reputation is shared and can impact your email deliverability, leading to issues like SendGrid emails going to spam on Outlook.com.
- List hygiene: Maintaining a clean email list by regularly removing inactive or invalid addresses is paramount. This preserves sender reputation and helps avoid spam traps.
- Authentication checks: Ensure your SPF, DKIM, and DMARC records are correctly set up and aligned. Failures in these areas are common reasons for emails being flagged as spam or phishing. You can check your authentication status to identify misconfigurations.
- Monitoring deliverability: Proactive monitoring of your email deliverability metrics and blocklist status can help detect and address issues before they escalate, preventing widespread delivery failures.
Expert view
Expert from Email Geeks sought clarification on whether the deliverability issues were occurring with an Office 365 hosted domain or a consumer Outlook.com account. This distinction is crucial for diagnosis, as corporate environments might have different security policies and configurations compared to personal accounts. Identifying the specific environment helps narrow down potential causes and appropriate troubleshooting steps.
Expert view
Expert from Email Geeks asked for the domain of the problematic tracking pixel. This inquiry is critical because the domain's reputation is directly tied to the deliverability of the email. If the pixel's domain is associated with a poor reputation or known malicious activity, it can cause the entire email to be flagged, even if the primary sending domain is clean.
What the documentation says
Official documentation from major email providers and internet standards bodies outlines the technical requirements and best practices for email deliverability. They detail the intricate layers of protection against spam and phishing, emphasizing the critical role of authentication protocols and sender reputation. Compliance with these guidelines is not merely suggested but often mandatory for consistent inbox placement. Understanding these foundational principles is key to navigating the complex landscape of email delivery.
Key findings
- Multi-factor email legitimacy: Microsoft determines email legitimacy through a combination of sender reputation, content analysis, and the proper implementation of authentication protocols (SPF, DKIM, DMARC).
- DMARC's role: DMARC is crucial for senders to indicate that their emails are protected by SPF and/or DKIM, and it provides receivers with mechanisms to report on and control unauthenticated mail.
- Blocklisting triggers: IP address and domain blocklisting occur when patterns of abusive sending are detected, directly impacting deliverability to major email providers.
- Engagement impact: Consistent low engagement rates, high complaint rates, and sending to invalid addresses significantly harm sender reputation and increase spam folder placement, as highlighted by various postmaster guidelines.
- Layered protection: Services like Microsoft 365 Exchange Online Protection (EOP) use multiple layers of protection, including connection, spam, and malware filtering, to block malicious or unwanted email.
Key considerations
- Implement comprehensive authentication: Adhere strictly to SPF, DKIM, and DMARC standards. Ensuring their correct configuration and alignment is fundamental for establishing trust with mailbox providers and preventing spoofing, which is a key component of advanced email authentication.
- Monitor sender reputation metrics: Actively monitor feedback loops, complaint rates, and engagement. Low engagement or high complaints will negatively impact your sender reputation, as explained in our guide on why emails fail.
- Maintain clean mailing lists: Regularly clean your email lists to remove inactive or invalid addresses to avoid bounces and spam traps, which can lead to your domain or IP being added to an email blacklist or blocklist.
- Content best practices: Adhere to content guidelines provided by ISPs, avoiding spammy keywords, suspicious links, or deceptive practices that could trigger phishing filters. This also includes vigilance over transactional email content.
- Understand ISP-specific nuances: Each ISP has unique filtering mechanisms. Familiarize yourself with postmaster guidelines from major providers like Microsoft to optimize deliverability for their users.
Technical article
Microsoft documentation outlines that the legitimacy of an email is assessed based on a variety of factors. These include a sender's established reputation, a thorough analysis of the email's content, and the correct implementation of authentication protocols such as SPF, DKIM, and DMARC. All these elements collectively contribute to an email's trustworthiness, influencing whether it reaches the inbox or is flagged as suspicious.
Technical article
RFC 8601 documentation clarifies the role of DMARC in email authentication. It allows a sender to explicitly declare that their emails are protected by SPF and/or DKIM, providing a strong signal of legitimacy to receiving servers. Furthermore, DMARC offers receivers a standardized mechanism to report on and control the disposition of mail that fails these authentication checks, enhancing security and trustworthiness across the email ecosystem.
Related resources
9 resources
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