Why are Microsoft email open rates low during IP warm-up?
Matthew Whittaker
Co-founder & CTO, Suped
Published 3 Jul 2025
Updated 16 Aug 2025
8 min read
Starting a new IP warm-up can be a challenging process, especially when dealing with major mailbox providers. Many email senders experience lower open rates on Hotmail and Outlook.com domains during this critical phase, even after gradually increasing volume to engaged subscribers. This often leads to questions about whether to continue or adjust the strategy.
The perception that Microsoft is stricter than other Internet Service Providers (ISPs) holds true for many. While your emails might be performing well with Google or Yahoo, Microsoft's filtering systems can be particularly sensitive to new or unfamiliar sending patterns. This makes their domains a unique challenge during any IP or domain warm-up process.
Understanding why this happens is the first step towards improving your deliverability. It typically comes down to how Microsoft's algorithms assess sender reputation and their stringent approach to preventing unwanted mail.
Microsoft's stricter filtering and reputation building
Microsoft, like other major mailbox providers, places a strong emphasis on sender reputation. When you begin sending from a new IP address, it lacks any established history, meaning it doesn't have a positive (or negative) reputation yet. Any sudden increase in email volume from a new or cold IP is immediately flagged as suspicious, often leading to emails being throttled, blocked (or blocklisted), or routed to the junk folder instead of the inbox. This directly impacts your open rates, even if your content is legitimate and your list is clean.
Their filters are designed to protect users from spam and phishing attempts. A new IP address needs to prove its legitimacy by demonstrating consistent, wanted email sending behavior over time. If the perception of unwanted mail is triggered, your open rates will plummet because the emails aren't reaching the primary inbox. This behavior is particularly pronounced with Microsoft, which is known for being quicker to block (or blacklist) mail from new or poorly performing IPs. This contributes to low deliverability and open rates on Hotmail and Outlook.
The primary goal of IP warm-up is to gradually build a positive sender reputation. This means starting with very small volumes to highly engaged segments of your list and slowly increasing the volume over days or weeks. However, even with a careful warm-up schedule, Microsoft's algorithms may still initially place your emails in the junk folder or block them, leading to the observed low open rates. This can also explain why new IPs might remain red in Microsoft SNDS during warming.
Best practice: dedicated IP warm-up strategy
When warming up a dedicated IP, it's crucial to begin with very low volumes of highly engaged subscribers. Slowly increase the volume over time, ensuring you monitor key metrics closely. This gradual approach allows mailbox providers to build trust in your sending patterns and prevents your emails from being flagged as suspicious. Consistency and patience are key for establishing a strong sender reputation.
Key metrics and tools for Microsoft warming
During IP warm-up, closely monitoring your key email metrics is paramount. While low open rates are a symptom, they are often a result of underlying deliverability issues such as high bounce rates or spam complaints. A sudden increase in bounce rates or a spike in spam complaints can severely damage your sender reputation, making it even harder to reach the inbox. This is especially true if you notice that emails are going to junk even with good metrics.
Microsoft's Smart Network Data Services (SNDS) is an invaluable tool for understanding your IP's reputation with Microsoft. Even if your IP shows as red in SNDS during warm-up, it provides data on spam complaints, junk mail placement, and other insights. While SNDS data might sometimes contradict deliverability during this phase, it’s still a key indicator.
Ensuring you have a dedicated Pointer (PTR) record configured for your sending IP address is also important. A PTR record provides the reverse DNS lookup, allowing recipients' servers to verify that the IP address is authorized to send email from your domain. This adds a layer of trust and can positively influence Microsoft's filtering decisions. Many senders find that a dedicated PTR is a better approach for Microsoft's warm-up process.
Expect lower initially, aim for gradual improvement.
Bounce rate
High rates harm sender reputation; soft bounces common during throttling.
Keep under 2-3%, address hard bounces immediately.
Spam complaint rate
Crucial negative signal; can lead to immediate blocklisting.
Maintain below 0.1%, ideally much lower during warm-up.
Strategic adjustments for Microsoft deliverability
To combat low open rates with Microsoft during IP warm-up, strategic adjustments are often necessary. One key strategy is to ensure your initial sending during warm-up is to your most engaged subscribers. These are contacts who consistently open and click your emails, providing positive engagement signals that Microsoft's algorithms value. Gradually increase your volume, always prioritizing engagement over sheer quantity.
Microsoft also offers a preemptive accommodation process, where you can proactively submit a request to inform them about your new sending IP. While not a guaranteed fix, it can sometimes expedite the trust-building process and help resolve IP warming issues. You can find more information about this process directly on Microsoft's support site. Some senders report significant improvement after their request is approved, while for others, the impact might be delayed.
Beyond volume and engagement, ensure your email content is clean, relevant, and avoids common spam triggers. Maintain a healthy email list by regularly removing inactive subscribers and managing bounces effectively. High bounce rates and spam complaints can lead to your IP being added to a blocklist (or blacklist), which significantly hinders deliverability. If you find your deliverability issues persist after being blocked, a comprehensive review of your sending practices is vital.
Common warm-up pitfalls
Sending too quickly: Rapidly increasing email volume from a new IP, especially to less engaged recipients.
Ignoring engagement: Not prioritizing active subscribers, leading to low opens and clicks.
Poor list hygiene: Sending to old, unverified, or purchased lists, increasing bounce and complaint rates.
Inconsistent volume: Erratic sending patterns rather than a smooth, gradual ramp-up.
Effective warm-up strategies
Gradual ramp-up: Start with small volumes and slowly increase daily sending, often over several weeks.
Engaged audience first: Send to your most active and responsive subscribers initially to generate positive signals.
Monitor feedback: Actively use tools like Microsoft SNDS to track reputation and adjust as needed.
Authentication setup: Ensure SPF, DKIM, and DMARC are correctly configured from day one.
Navigating persistent low open rates
Even with optimal strategies, it's common for Microsoft open rates to lag behind those of other major mailbox providers like Google and Yahoo. This isn't always a sign of failure but can often be the new normal for many senders. If your rates aren't getting worse and you're seeing gradual improvement, continuing your planned warm-up schedule is often the best course of action.
Patience is a virtue in IP warming, especially with Microsoft. While some senders see improvement within a few weeks, others report it taking anywhere from five weeks to three months for Microsoft open rates to align with other ISPs, or for them to simply stabilize at a slightly lower baseline. The key is consistent monitoring and not overreacting to minor fluctuations.
If, however, your Microsoft metrics worsen (e.g., increasing spam complaints or bounces), then a temporary adjustment is warranted. This might involve freezing your sending volume to Microsoft domains, or even slightly reducing it, to allow your reputation to recover before attempting to increase again. Remember, the goal is to build trust, and sending more emails when your reputation is suffering will only exacerbate the problem. For more comprehensive insights, refer to general guidance on improving overall email deliverability.
A critical foundational element for successful warm-up is robust email authentication. SPF, DKIM, and DMARC are essential for verifying your sender identity and signaling legitimacy to mailbox providers. Ensure these are correctly implemented and monitored, as issues with authentication can severely impact your deliverability and contribute to low open rates. Improper authentication is a common reason why your emails might not reach the inbox, even during a dedicated IP warming phase.
Maintain consistent, gradual increases in sending volume based on positive engagement.
Prioritize sending to your most engaged subscribers first during the initial warm-up phase.
Actively use Microsoft's SNDS to monitor your IP's reputation and identify potential issues.
Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned.
Common pitfalls
Sending too high a volume too soon, triggering spam filters and blocklists.
Ignoring low engagement metrics and continuing to increase sending volume.
Failing to clean your email list, leading to high bounce rates and spam trap hits.
Not monitoring Microsoft SNDS, missing critical reputation warnings.
Expert tips
Even after warm-up, Microsoft open rates might remain slightly lower than other ISPs, which is often the new normal.
If deliverability worsens, pause or slightly reduce volume before continuing to ramp up.
Focus on content quality and relevance to encourage positive subscriber engagement.
Consider a dedicated PTR record for your sending IP to enhance trust with Microsoft.
Marketer view
A marketer noticed significantly lower open rates on Microsoft domains like Hotmail and Outlook.com during IP warm-up compared to other mailbox providers, despite gradually increasing volume to engaged subscribers.
2022-05-10 - Email Geeks
Expert view
An expert highlighted that Microsoft is considerably stricter than other ISPs, and lower overall open rates at Microsoft are now a common occurrence for many senders.
2022-05-10 - Email Geeks
Key takeaways
Experiencing low open rates with Microsoft during IP warm-up is a common hurdle for email senders. This is primarily due to Microsoft's stringent filtering, which prioritizes established sender reputation and positive engagement. New IPs lack this trust, making them susceptible to junk folder placement or outright blocking (or blocklisting).
Successfully navigating this period requires a combination of patience, diligent monitoring of metrics (especially via SNDS), adherence to best practices like sending to engaged users, and considering proactive measures like preemptive accommodation requests. While some lag in Microsoft open rates might become the new normal, a consistent and strategic approach will ultimately help you establish the necessary trust for improved deliverability over time.
Hotmail
and 
Outlook.com domains during this critical phase, even after gradually increasing volume to engaged subscribers. This often leads to questions about whether to continue or adjust the strategy.
Google
or Yahoo, Microsoft's filtering systems can be particularly sensitive to new or unfamiliar sending patterns. This makes their domains a unique challenge during any IP or domain warm-up process.
