How to fix low open rates in Outlook due to spam filtering and sender reputation issues?
Matthew Whittaker
Co-founder & CTO, Suped
Published 11 May 2025
Updated 16 Aug 2025
7 min read
Dealing with low open rates in Outlook can be a frustrating experience, especially when your emails are consistently landing in the spam or junk folder. It often signals underlying issues with spam filtering and, more significantly, your sender reputation. Many senders find their emails perform well with other mailbox providers but struggle specifically with Outlook.com.
This guide will walk you through the common causes behind these deliverability challenges and provide actionable strategies to diagnose and fix them, ultimately helping you improve your email open rates and ensure your messages reach the inbox.
Understanding Outlook's filtering mechanisms
Microsoft's email filtering mechanisms, used by Outlook and Hotmail, are known for their strictness compared to other mailbox providers. They employ sophisticated algorithms to assess incoming mail, heavily relying on the sender's reputation (often called domain reputation or IP reputation) to determine inbox placement. A strong reputation means your emails are more likely to reach the inbox, while a poor one often leads to the junk folder or even outright rejection.
Your sender reputation is a dynamic score influenced by numerous factors. These include the volume and consistency of your sending, your bounce rates, and perhaps most critically, recipient engagement. Low open rates, high unsubscribe rates, and frequent spam complaints negatively impact this score. Mailbox providers interpret these signals as indicators of low-quality or unwanted mail.
To gain visibility into your sender reputation with Microsoft, it is essential to utilize Smart Network Data Services (SNDS). This free tool provides crucial data on your IP and domain health, including spam complaint rates and blocklist (or blacklist) status, which can help pinpoint specific issues.
A common scenario for low Outlook open rates is when emails are delivered to the junk folder without the sender even realizing it. Because these emails are technically delivered, they don't bounce, making the problem harder to detect without careful monitoring or direct feedback from recipients. This hidden deliverability issue can severely impact your campaigns. For more insights into this specific problem, read why your emails go to the junk folder.
Diagnosing the drop in open rates
Many deliverability problems with Outlook begin at the list acquisition stage. High volumes of low-quality signups, often from bots or uninterested users, can quickly dilute your list quality. These contacts are unlikely to engage with your emails, leading to low open rates and potentially high spam complaints, which in turn severely damage your sender reputation. Addressing this at the source is critical.
Unengaged subscribers are a silent killer of email deliverability. If a significant portion of your list (especially those using Microsoft domains) consistently ignores your emails, it signals to mailbox providers that your content is not valued by recipients. This lack of engagement directly contributes to a declining sender reputation, leading to more emails being flagged as spam. For a broader understanding of why your open rates might be low, refer to our detailed guide.
Signs of a problem
Abnormally low open rates: Your open rates for Outlook/Hotmail addresses are significantly lower than other domains.
Emails found in junk: Recipients report finding your legitimate emails in their spam folder.
Increased bounce rates: A rise in bounces, particularly soft bounces, can indicate content or reputation issues.
Underlying causes
Poor list hygiene: Sending to invalid, inactive, or unengaged email addresses.
Low-quality signups: Bot activity or fraudulent signups inflating your list with non-engaging contacts.
Sender reputation decline: Negative feedback loops, spam complaints, or sending to spam traps.
Another area to investigate is whether your sending IP or domain is listed on any email blocklists (or blacklists). While not all blocklists are weighted equally by Microsoft, being listed on a major one like Spamhaus can certainly impact your deliverability to Outlook. You can use a blocklist checker to quickly see if your domain or IP is listed. Remember, an email blacklist is essentially a real-time database of IP addresses or domains that have been identified as sources of spam. Being listed can prevent your emails from reaching their intended recipients.
Strengthening your sender reputation
Robust email authentication is the bedrock of good deliverability. Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. Microsoft places significant emphasis on these protocols to verify sender legitimacy. Any misconfiguration can lead to emails being marked as suspicious or outright rejected. For a simplified explanation of these protocols, check out our guide on DMARC, SPF, and DKIM. If you're encountering issues specifically with Microsoft's spam filters, proper authentication is your first line of defense.
Implementing a double opt-in process for all new subscribers is one of the most effective ways to ensure list quality. It verifies that subscribers genuinely want to receive your emails and helps filter out bots or mistyped addresses. This practice significantly reduces bounce rates and spam complaints, positively impacting your sender reputation.
Implementing robust signup practices
CAPTCHA and honeypots: Incorporate these on your signup forms to deter automated bot signups, which often result in inactive or spam trap addresses.
Email verification: Use a real-time email verification service at the point of signup to validate addresses before they even join your list.
Clear consent: Ensure users explicitly opt-in and understand what kind of emails they will receive.
Regular list hygiene is crucial. Segment your list to identify and suppress unengaged Outlook contacts. Continuing to send emails to inactive subscribers drags down your overall engagement metrics and signals negative behavior to Microsoft. Removing these contacts, even temporarily, can significantly improve your sender reputation and deliverability.
Strategies for recovery and ongoing maintenance
Once you've addressed the foundational issues, actively suppressing unengaged Outlook (or Hotmail) contacts can be a powerful step in reputation recovery. This means refraining from sending promotional emails to these segments, even if you don't remove them from your database entirely. This practice helps to improve engagement rates among your active subscribers, which is a key signal for Microsoft that your emails are valued. For guidance on recovering your email domain reputation, consult our comprehensive guide.
If your sender reputation has been severely damaged, a gradual IP or domain warm-up strategy might be necessary. This involves sending emails in small, increasing volumes to highly engaged subscribers first, then gradually expanding to a wider audience. This slow and steady approach helps rebuild trust with mailbox providers, including Microsoft, by demonstrating consistent positive sending behavior.
Ongoing monitoring is not optional. Keep a close eye on your engagement metrics, specifically open rates and spam complaint rates, especially for your Outlook subscribers. Most mailbox providers, including Outlook and Gmail and Yahoo, have strict spam rate thresholds (often below 0.3%). Staying below these limits is crucial for maintaining good standing and avoiding the junk folder. Regularly verifying your email list for invalid addresses and spam traps is also critical to reducing bounce rates.
Metric
Target
Impact
Open rate
> 10-15% (or higher for engaged segments)
Low open rates signal disengagement, harming sender reputation. High rates indicate content relevance and positive reception.
Spam complaint rate
< 0.1% (ideally < 0.02%)
High complaint rates are a strong negative signal, leading to junk folder placement and blocklist (or blacklist) listings.
Bounce rate
< 2% (hard bounces should be near 0%)
High bounce rates, especially hard bounces, damage sender reputation and indicate poor list quality.
Engagement (clicks, replies)
Consistent and growing activity
Positive engagement signals value to Microsoft, improving inbox placement. Low engagement indicates disinterest.
Restoring your Outlook deliverability
Fixing low open rates in Outlook due to spam filtering and sender reputation issues requires a methodical and persistent approach. It is not a quick fix, but rather a commitment to email best practices.
By focusing on stringent list hygiene, implementing robust authentication, closely monitoring your metrics, and actively managing recipient engagement, you can gradually rebuild and maintain a strong sender reputation with Microsoft. This will lead to improved inbox placement and, consequently, higher open rates for your Outlook subscribers.
Views from the trenches
Best practices
Implement double opt-in for all new subscribers to ensure high-quality, engaged contacts.
Utilize CAPTCHA and honeypot solutions on your signup forms to deter bot-generated signups effectively.
Actively monitor your sender reputation using tools like Microsoft SNDS to identify and address issues early.
Regularly segment and suppress inactive subscribers, especially those on Microsoft domains, to improve engagement metrics.
Prioritize adhering to strict email authentication standards, including SPF, DKIM, and DMARC, for all outgoing mail.
Common pitfalls
Neglecting to implement double opt-in, leading to a high volume of unengaged or bot-generated signups.
Failing to monitor key deliverability metrics, such as complaint rates and bounce rates, for early warning signs.
Assuming all blocklist (or blacklist) listings directly impact Microsoft deliverability without verifying their influence.
Continuously sending emails to disengaged subscribers, which negatively affects your sender reputation over time.
Overlooking the importance of email content quality and personalization, which impacts recipient engagement.
Expert tips
Focus on increasing engagement by sending relevant, valuable content to your subscribers.
Guide recipients to mark your emails as “not junk” and add your address to their safe senders list.
For significant issues, reach out to Microsoft support through their dedicated sender support channels.
Understand that Microsoft's filtering can be more aggressive than other mailbox providers, requiring a tailored approach.
Consistently clean your email list by removing hard bounces and addressing soft bounces promptly.
Expert view
Expert from Email Geeks says implementing double opt-in can automatically clean your email list and prevent future issues.
March 20, 2023 - Email Geeks
Expert view
Expert from Email Geeks says employing a CAPTCHA and honeypot on signup forms is essential to combat bot activity.
