Why has Microsoft email deliverability suddenly decreased and how can it be fixed?

Key findings

• Widespread issue: Multiple senders reported a sudden decline in Microsoft deliverability, often around the same timeframe (e.g., late January), indicating a systemic shift rather than isolated incidents.
• Engagement vs. deliverability: Even when emails technically 'inbox,' engagement metrics (like open rates) may plummet, suggesting placement in less prominent parts of the inbox or passive filtering.
• Microsoft algorithm changes: There's speculation that Microsoft has recently implemented new technical changes, potentially tightening filtering for senders perceived as borderline, leading to increased junk folder placement.
• Volume sensitivity: Even modest increases in sending volume (e.g., 1.5x original volume) can negatively impact Microsoft deliverability, especially for seasonal senders or those with inconsistent volumes.
• Authentication impact: Proper email authentication (SPF, DKIM, DMARC) is increasingly critical, as Microsoft (and other providers) are slaming the gate on unverified senders.

Key considerations

• Review data sources: Distinguish between deliverability data from probe accounts versus Microsoft SNDS, as each provides different insights into inbox placement.
• Enhance list hygiene: Rigorously clean email lists, removing inactive users and ensuring all recipients are genuinely opted-in to improve overall domain reputation.
• Content and infrastructure review: Examine email content for potential spam triggers, and check infrastructure for issues like shared IPs or List-Unsubscribe domains that could be causing problems.
• Monitor headers: Regularly check email headers, particularly X-Microsoft-Antispam values like BCL (Bulk Complaint Level) and SCL (Spam Confidence Level), to understand how Microsoft classifies your mail.
• Submit support tickets: If problems persist after implementing fixes, consider submitting a ticket directly to Hotmail (Microsoft Outlook.com Postmaster team) for further investigation.

Key opinions

• Shared experience: Many marketers noted experiencing a sudden drop in Microsoft deliverability around the same time, suggesting a widespread issue rather than an isolated one.
• Content concerns: Content-related issues are increasingly causing deliverability problems, leading to emails being 'clapped' (filtered) by providers.
• Seasonal volume sensitivity: Microsoft (and Hotmail) can be particularly sensitive to large or inconsistent seasonal spikes in sending volume, which can lead to reputation damage.
• Inconsistency in internal scores: Sometimes, emails are still caught in the junk folder even when internal headers (like BCL) indicate a low spam score, pointing to other underlying issues.

Key considerations

• Proactive monitoring: Regularly monitor inbox placement data and email engagement metrics, not just delivery rates, to catch subtle shifts in deliverability.
• Address content issues: Review email content for elements that might trigger spam filters, focusing on adding value and avoiding overly promotional or irrelevant material, as unwanted content can lower sender reputation.
• Manage volume carefully: If experiencing deliverability issues after a volume increase, consider scaling back and slowly re-warming your sending reputation, especially for throttled Microsoft domains.
• Seek specific assistance: If internal data shows good inboxing but engagement is low, it's often a sign that a more nuanced look at your campaign (or Microsoft's filtering) is needed.
• Diagnose through comparison: Compare current email performance with past successful campaigns to identify subtle changes in content, links, or sending patterns that may have contributed to the decline.

Key opinions

• Data source matters: The interpretation of deliverability data (e.g., from Validity's probe accounts vs. Microsoft's SNDS) dictates the diagnostic approach.
• Hidden reputation factors: External links or Content Delivery Networks (CDNs) can unexpectedly impact sender reputation and cause delivery issues, even if they aren't directly related to the email infrastructure.
• Microsoft's constant tuning: Microsoft frequently tunes and ratchets its filtering systems, which can cause issues for senders who are 'borderline' in terms of their reputation or practices.
• BCL isn't always the full story: A low BCL (Bulk Complaint Level) score, indicating non-spam, doesn't always guarantee inbox placement, as other factors can still lead to junking.

Key considerations

• Thorough data hygiene: An accumulation of poor data or non-opt-in addresses on your list can slowly shift mail from the inbox to the spam folder, necessitating a robust approach to list quality.
• Infrastructure audit: Evaluate your sending infrastructure for potential issues, including whether you are on a shared IP address or using a shared List-Unsubscribe domain, which can affect reputation.
• Content and domain analysis: Examine both the specific email content and the broader sending domain for issues. Even subdomains can be affected if the primary domain's reputation is poor.
• Understand Microsoft's policies: Stay informed about Microsoft's evolving sender requirements and filtering policies. They are constantly adapting to combat spam and protect users.
• Differentiate mail streams: Separate different types of mail (transactional, marketing, invoicing) onto different subdomains or IPs if possible, to prevent one stream from negatively impacting another, particularly with Microsoft's stricter filtering.

Key findings

• Authentication as a baseline: Industry standards and Microsoft documentation consistently emphasize the critical need for correctly configured SPF, DKIM, and DMARC records for sender verification.
• Reputation is key: Mailbox providers, including Microsoft, heavily rely on sender IP and domain reputation, which is built over time based on sending practices and recipient interaction.
• Engagement signals: Positive recipient engagement (opens, clicks, replies, not marking as spam) is a primary driver for inbox placement, while negative engagement (spam complaints, low opens) harms reputation.
• SNDS data: Microsoft's Smart Network Data Services (SNDS) provides senders with data on their IP reputation, complaint rates, and other metrics crucial for troubleshooting.

Key considerations

• Implement DMARC: Go beyond SPF and DKIM by implementing DMARC to enforce authentication and receive valuable aggregate and forensic reports, which aid in diagnosing authentication failures.
• Monitor blocklists: Regularly check both public and private blacklists (blocklists) to ensure your IP or domain has not been listed, as this can severely impact deliverability.
• Adhere to sending limits: Respect recipient mailbox provider sending limits and throttling rules. Rapid, unmanaged volume increases can trigger filtering mechanisms.
• Feedback loop participation: Enroll in Microsoft's JMRP (Junk Mail Reporting Program) to receive reports on users who mark your email as junk, allowing for prompt list cleaning.
• Optimize for engagement: Focus on sending relevant, desired content to an engaged audience to foster positive interactions and improve your sender reputation. A good example is optimizing your inbox placement.