Why are my emails not delivering to Outlook and being flagged as phishing?
Matthew Whittaker
Co-founder & CTO, Suped
Published 29 Apr 2025
Updated 16 Aug 2025
7 min read
It can be incredibly frustrating when your emails suddenly stop reaching Outlook inboxes and, even worse, are flagged as phishing attempts. This isn't just an inconvenience, it's a significant barrier to effective communication and can severely impact your business operations. I've seen many organizations struggle with this exact problem, especially with Microsoft's rigorous filtering systems.
When emails get flagged as phishing, it usually means that a recipient's email provider, like Outlook or Office 365, has detected characteristics commonly associated with malicious emails. This can range from suspicious links and content to broader issues with your sender reputation or email authentication.
Sender reputation and blacklists
One of the most significant factors influencing whether your emails land in the inbox or are flagged as spam (or even phishing) is your sender reputation. This reputation is built over time based on your sending practices, the engagement of your recipients, and whether your IP addresses or domains have been implicated in sending unwanted emails. If your domain or IP has a poor reputation, Outlook's filters are far more likely to be suspicious of your messages.
A common cause for a damaged reputation is being listed on a email blocklist. These blacklists are databases of IP addresses and domains known to send spam or malicious content. Even if your sending is legitimate, being on a shared IP address or using a third-party service that also sends problematic emails can get you listed. It's also possible that your domain itself has been compromised or misused, leading to a listing. If you are experiencing issues, one of the first things I recommend is to check your blocklist status.
Microsoft, in particular, has sophisticated filtering mechanisms that closely monitor sender behavior and domain reputation. They utilize a complex system to assign a Spam Confidence Level (SCL) and Bulk Complaint Level (BCL) to incoming emails, which determines their fate. A low reputation can lead to your emails being marked as junk, quarantined, or outright rejected, often with a phishing warning, even if your content seems innocuous. This is why it's vital to focus on maintaining a pristine sender reputation.
Authentication is key
Email authentication protocols are non-negotiable for reliable delivery to Outlook and other major inboxes. SPF, DKIM, and DMARC are essential for proving that you are a legitimate sender and that your emails haven't been tampered with. Without proper authentication, your emails are far more likely to be viewed as suspicious, leading to deliverability issues and phishing warnings.
Microsoft has been particularly vocal about the importance of these standards. For instance, if your SPF (Sender Policy Framework) record isn't correctly configured, it could lead to your emails being rejected or flagged. Similarly, a broken DKIM (DomainKeys Identified Mail) signature, which verifies that the email content hasn't been altered in transit, will raise red flags. Diagnosing DKIM temporary error rates with Microsoft is a common task for deliverability professionals.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) builds upon SPF and DKIM by allowing you to tell receiving servers how to handle emails that fail authentication and to receive reports on email authentication failures. Implementing a DMARC policy, even a relaxed one, is crucial for demonstrating your commitment to email security and preventing your domain from being used for phishing.
Email authentication checklist
Ensuring your email authentication records are set up correctly is fundamental to good deliverability and avoiding phishing flags. Here’s a quick checklist:
SPF record: Verify your SPF record includes all legitimate sending IP addresses and services. An invalid record can lead to soft fails or rejections.
DKIM signatures: Confirm that your emails are consistently signed with a valid DKIM signature. Errors here often result in emails being marked as unverified.
DMARC policy: Implement a DMARC record to protect your domain from unauthorized use and gain visibility into your email ecosystem.
BIMI records: For enhanced brand visibility and trust, consider adding a BIMI record, though this is not strictly for deliverability.
Content and engagement
The content of your email itself plays a crucial role in whether it's flagged as spam or phishing. Email filters scan for suspicious keywords, unusually formatted links, hidden text, and other elements commonly found in fraudulent messages. If your email contains a link to an unfamiliar or untrusted domain, or if a tracking pixel's domain has a bad reputation, it can immediately trigger a phishing warning. I've seen instances where a single problematic tracking pixel caused entire campaigns to be blocked.
Beyond explicit phishing indicators, general 'spammy' content can also harm your deliverability. This includes excessive use of exclamation marks, all caps, overly promotional language, or broken HTML. Email providers like Outlook are increasingly sophisticated in detecting subtle cues that might indicate unwanted email. Your email content should be clean, relevant, and free of anything that could be misinterpreted.
User engagement also heavily influences content filtering. If recipients consistently mark your emails as spam, delete them without opening, or show very low interaction, Outlook learns that your emails are not valued. This negative feedback loop can lead to legitimate emails being flagged. It's not just about avoiding explicit spam triggers, but also about building a positive relationship with your subscribers through valuable content and managing your email deliverability.
Technical nuances and ongoing monitoring
Sometimes, the issue isn't obvious reputation or content, but subtle technical details. In one specific case, a deprecated conversion tracking pixel from a third-party service began triggering a 'PHISH' flag from Microsoft. This pixel, which had been in use for a long time without issues, suddenly caused delivery failures. Removing it immediately resolved the problem. This highlights how continuously evolving spam and phishing detection algorithms can suddenly identify previously innocuous elements as threats.
Another technical aspect to consider is how your emails are sent and processed, particularly internally. BCCing internal inboxes, for example, for archiving or database logging, can sometimes lead to your sending server hitting rate limits, which Outlook might interpret as a sign of problematic sending behavior. This can inadvertently affect your overall sender reputation and cause deliverability hiccups for all your emails, not just the internal ones.
Given Outlook's strict filtering, continuous monitoring and proactive adjustments are essential. This includes regularly reviewing your email logs for bounce messages, keeping an eye on your sender reputation, and being prepared to swiftly address any new warnings or blocks. Ignoring these technical signals can lead to prolonged deliverability issues and a damaged brand image.
Resolving outlook deliverability issues
Common issues
Poor sender reputation: Low engagement rates, high spam complaints, or being on a blacklist.
Authentication failures: Incorrect or missing SPF, DKIM, or DMARC records.
Suspicious content: Spammy keywords, broken links, hidden text, or problematic tracking pixels. One Microsoft support article specifically warns about phishing and suspicious behavior.
Recipient actions: Recipients frequently marking your emails as spam.
Effective solutions
Improve sender reputation: Send wanted content, clean your list, and engage with Outlook Sender Support.
Optimize content: Remove suspicious links, use clear language, and avoid spam triggers. Conduct regular email deliverability tests.
Monitor engagement: Encourage positive interactions and promptly address complaints or low engagement.
If you are struggling with Outlook filtering your emails into junk or flagging them as phishing, the path to resolution involves a combination of technical fixes, content optimization, and reputation management. Start by verifying your authentication records, then scrutinize your email content for any elements that could be misconstrued as malicious. Finally, assess your sender reputation and address any issues that could be causing Outlook's filters to mistrust your emails. Proactive monitoring and adherence to best practices are key to ensuring your messages reach their intended recipients.
Remember, email deliverability is an ongoing effort. The digital landscape and filtering algorithms are constantly evolving, so what works today might need adjustments tomorrow. By staying vigilant and adopting a comprehensive approach, you can significantly improve your chances of consistently landing in the Outlook inbox and avoiding those dreaded phishing warnings.
Views from the trenches
Best practices
Regularly audit all external links and tracking pixels embedded in your emails to ensure they are reputable and not deprecated.
Ensure all email authentication records, including SPF, DKIM, and DMARC, are correctly configured and actively monitored for compliance.
Continuously monitor your domain and IP reputation using postmaster tools and blocklist services to catch issues early.
Maintain a clean and engaged subscriber list by removing inactive users and encouraging positive interactions to improve sender signals.
Common pitfalls
Using deprecated or unknown tracking pixel domains that can trigger phishing flags from sophisticated filters like Microsoft's.
Not segmenting email traffic, leading to shared reputation issues when using third-party sending services with other problematic senders.
Ignoring bounced emails or encountering rate limits, as these can negatively impact your overall sender score and deliverability.
Overlooking subtle aspects of email content, including hidden elements, which can inadvertently trigger spam filters and phishing warnings.
Expert tips
Verify that any third-party services used for email content, tracking, or advertising do not have a compromised or poor reputation.
Implement a DMARC policy at 'p=quarantine' or 'p=reject' to protect your domain from spoofing and enhance recipient trust.
Actively engage with Microsoft's sender support services if you face persistent delivery challenges to Outlook and Office 365 inboxes.
Regularly review email engagement metrics, such as open and click-through rates, as low engagement can contribute to emails being flagged as spam.
Marketer view
We encountered sudden deliverability issues with Outlook accounts, where even internal proofs were not being received, suggesting a recent update might be affecting promotional email filtering.
April 27, 2021 - Email Geeks
Expert view
A specific tracking pixel domain was being flagged by Microsoft as 'PHISH', and removing it resolved the email delivery problem, indicating a change in Microsoft's detection of such assets.
Outlook inboxes and, even worse, are flagged as phishing attempts. This isn't just an inconvenience, it's a significant barrier to effective communication and can severely impact your business operations. I've seen many organizations struggle with this exact problem, especially with Microsoft's rigorous filtering systems.
