What are the best practices for improving email deliverability to Microsoft Outlook and Hotmail?
Michael Ko
Co-founder & CEO, Suped
Published 31 May 2025
Updated 19 Aug 2025
7 min read
Many senders struggle to reach the inbox of Microsoft's email services, including Outlook and Hotmail. These platforms have some of the most robust spam filtering systems, often leading to legitimate emails landing in the junk folder or being blocked entirely.
Improving your deliverability to these mailboxes requires a proactive and comprehensive strategy, focusing on building and maintaining a strong sender reputation and adhering to strict technical and content best practices.
Understanding email authentication and Microsoft's requirements
At the core of successful email delivery to Outlook and Hotmail lies proper email authentication. Microsoft, like other major mailbox providers, relies heavily on protocols like SPF, DKIM, and DMARC to verify the authenticity of incoming mail. Without these configured correctly, your emails are much more likely to be flagged as suspicious or spam.
Microsoft has notably tightened its requirements for high-volume senders, making the proper implementation of these authentication standards mandatory. Ensuring your email sending domain has valid SPF, DKIM, and DMARC records is no longer optional, it is a prerequisite for reliable inbox placement. This helps Microsoft's systems confirm that you are who you say you are, reducing the risk of your emails being impersonated or rejected.
SPF (Sender Policy Framework) specifies which mail servers are authorized to send email on behalf of your domain. DKIM (DomainKeys Identified Mail) provides a digital signature, allowing recipients to verify that an email was not altered in transit. DMARC (Domain-based Message Authentication, Reporting, and Conformance) builds upon SPF and DKIM, providing a framework for domain owners to instruct mailbox providers on how to handle emails that fail authentication and to receive reports. A simple guide to DMARC, SPF, and DKIM can help with this.
The power of DMARC
DMARC allows you to monitor your email streams, detect unauthorized sending, and ultimately tell receiving servers like Microsoft.com what to do with emails that fail authentication (e.g., quarantine or reject them). Setting up a DMARC record, even with a policy of p=none, is a crucial step towards better deliverability and preventing spoofing. For help, explore our DMARC record and policy examples.
Cultivating sender reputation and maintaining a healthy list
Your sender reputation is paramount. Microsoft uses complex algorithms to assess the trustworthiness of your sending IP and domain, taking into account various factors, including user engagement and complaint rates. A poor reputation means your emails will struggle to reach the inbox, often ending up in the junk folder or being blocklisted (blacklisted). To improve reputation, monitoring tools and list hygiene are essential.
Microsoft offers specific tools to help senders manage their reputation: Smart Network Data Services (SNDS) and the Junk Mail Reporting Program (JMRP). SNDS provides data on your IP reputation, spam complaint rates, and other metrics from Microsoft's perspective. JMRP allows you to receive copies of emails marked as spam by Outlook.com users. Enrolling in these programs gives you valuable insights to diagnose and address deliverability issues. You can find more information about these services on the Outlook.com website for senders.
Maintaining a clean and engaged email list is perhaps the most impactful practice. Sending to inactive or invalid email addresses can severely damage your sender reputation, increasing bounce rates and potentially leading to spam trap hits. Regularly scrubbing your list to remove unengaged subscribers and invalid addresses will significantly improve your overall deliverability, especially to Hotmail and Outlook inboxes.
Best practices for list hygiene
Implement double opt-in: Ensure subscribers explicitly confirm their interest.
Regularly remove inactive users: Clean your list of recipients who haven't opened or clicked in a long time.
Suppress bounced addresses: Immediately remove hard bounces to prevent further damage.
Avoid purchasing lists: Purchased lists often contain spam traps and invalid addresses.
Common pitfalls to avoid
Ignoring unengaged subscribers: Continuing to email recipients who don't open leads to low engagement and negative signals.
Not processing unsubscribes promptly: Delays can lead to spam complaints.
Sending to old or unverified lists: This increases bounce rates and spam trap hits.
Using single opt-in only: Increases the risk of invalid or unwanted subscriptions.
Content quality and careful sending practices
The content of your emails plays a significant role in how Microsoft's filters classify them. Generic, overly promotional, or poorly formatted emails are more likely to be flagged. Focus on creating valuable, relevant, and engaging content that encourages positive interactions like opens and clicks, and avoid elements commonly associated with spam. This also means ensuring all URLs within your email are valid and reputable.
When it comes to sending, consistency and a proper warm-up process are key, especially for new IPs or domains. Sending too many emails too quickly from a new or cold IP can trigger spam filters, regardless of content quality or authentication. Gradually increasing your sending volume (IP warming) allows mailbox providers to build a positive reputation for your sending infrastructure over time.
Furthermore, the infrastructure you use to send emails matters. Some cloud providers (like certain AWS IP ranges) may have shared IP addresses with a mixed reputation due to various users sending from them. For high-volume or critical email sending, consider using dedicated IP addresses and ensure your email service provider (ESP) maintains excellent IP hygiene across their network. More on email deliverability issues can be found in our guide.
Area
Recommendation
Why it matters for Microsoft
Content
Provide relevant, personalized, and engaging content.
Increases positive engagement signals (opens, clicks), which Outlook values highly.
URLs
Use valid, reputable URLs and avoid excessive or suspicious links.
Microsoft actively scans URLs. Bad or too many links can trigger filters.
Volume
Start with low volumes and gradually increase, especially for new IPs.
Helps build a positive sender reputation (IP warming) with Microsoft's systems.
IP types
Prefer dedicated IPs over shared cloud IPs for critical sending.
Gives you more control over your IP reputation, avoiding issues from other senders.
Continuous monitoring and effective troubleshooting
Email deliverability is not a one-time setup, it requires ongoing vigilance. Regularly monitor your email performance metrics, such as open rates, click-through rates, and bounce rates. Pay particular attention to complaint rates, as high complaint rates are a strong indicator of issues and can quickly lead to blocklisting (blacklisting) by Microsoft.
Understanding Microsoft's internal scoring mechanisms, such as Spam Confidence Level (SCL) and Bulk Confidence Level (BCL), can provide deeper insights. These scores, often found in email headers, indicate how likely Microsoft views an email as spam or bulk mail. A low SCL/BCL score is desirable, as it means Microsoft views your emails favorably. Higher scores indicate potential issues. You can refer to Microsoft.com's documentation on spam confidence levels for more detail.
When deliverability issues arise, prompt troubleshooting is crucial. Analyzing bounce messages provides specific reasons for non-delivery. Checking your domain and IP addresses against common blocklists (blacklists) is also a necessary step. Sometimes, issues can stem from compromised accounts sending spam, or from unexpected changes in your email content or sending patterns. For an in-depth look, see our guide on an in-depth guide to email blocklists. If you're encountering common DMARC issues with Microsoft 365, we have a dedicated article for that too.
Understanding SCL and BCL
Microsoft assigns a Spam Confidence Level (SCL) and Bulk Confidence Level (BCL) to incoming emails.
SCL indicates the likelihood of an email being spam, with higher numbers meaning higher spam likelihood.
BCL represents the likelihood of an email being bulk mail, where a higher score suggests it's marketing or promotional.
Both scores influence inbox placement. Monitoring them through message headers can help you diagnose deliverability problems.
Views from the trenches
Best practices
Always ensure recipients have explicitly opted-in and confirmed their desire to receive your emails.
Maintain a disciplined approach to IP warming when using new sending IPs or domains.
Regularly enroll and monitor insights from Microsoft's SNDS and JMRP programs.
Promptly remove all hard bounces and unengaged subscribers from your mailing lists.
Common pitfalls
Sending a large volume of emails immediately from new, cold IP addresses.
Failing to implement a feedback loop (FBL) to receive spam complaints.
Overlooking the importance of clear rDNS labeling for your sending infrastructure.
Relying solely on shared IP addresses from generic cloud providers for high-volume sends.
Expert tips
Expert from Email Geeks says: Using dedicated IP addresses can help isolate your sending reputation from other users.
Expert from Email Geeks says: Contacting Hotmail’s support directly can provide a customized list of actions to improve deliverability.
Expert from Email Geeks says: Examine URLs and domains embedded in your email content, as these can significantly impact inbox placement even with good BCL/SCL scores.
Marketer from Email Geeks says: Consistently testing your email content through various tools can reveal issues before sending to a live audience.
Expert view
Expert from Email Geeks says: Using dedicated IP addresses can help isolate your sending reputation from other users.
2023-08-10 - Email Geeks
Expert view
Expert from Email Geeks says: Contacting Hotmail’s support directly can provide a customized list of actions to improve deliverability.
2024-03-15 - Email Geeks
Ensuring email success with Microsoft
Improving email deliverability to Microsoft Outlook and Hotmail is a continuous effort that combines technical setup, reputation management, and strategic content creation. By prioritizing strong authentication (SPF, DKIM, DMARC), actively managing your sender reputation through Microsoft's tools, and diligently maintaining a clean and engaged subscriber list, you can significantly enhance your inbox placement.
Furthermore, paying close attention to your email content, adhering to responsible sending volumes, and troubleshooting issues proactively using metrics like SCL and BCL will cement your sending credibility. Embracing these best practices will help ensure your important messages consistently reach your recipients' inboxes.
Microsoft.com what to do with emails that fail authentication (e.g., quarantine or reject them). Setting up a DMARC record, even with a policy of p=none, is a crucial step towards better deliverability and preventing spoofing. For help, explore our DMARC record and policy examples.
