What is fundamentally different about warming up IP addresses for Microsoft domains?
Matthew Whittaker
Co-founder & CTO, Suped
Published 18 Apr 2025
Updated 19 Aug 2025
6 min read
Warming up a new IP address is a critical step for any sender looking to achieve strong email deliverability. The process involves gradually increasing your email volume over time, allowing mailbox providers to assess your sending habits and build a positive reputation for your IP.
However, if you've ever dealt with Microsoft domains, such as Outlook.com and Hotmail, you might notice that their approach to IP warming feels distinctly different. Many senders find that even following standard, cautious warm-up schedules, they still encounter persistent deliverability challenges with Microsoft.
The core question isn't whether Microsoft requires IP warming, but rather, what fundamental differences in their filtering systems necessitate a modified approach. It’s not simply about sending volume, but a deeper assessment of sender trustworthiness.
While other major providers like Gmail rely heavily on similar principles, Microsoft's particular weighting of various reputation signals, coupled with its proprietary internal systems, creates a unique landscape for senders. Understanding these nuances is key to successful IP warm-up and sustained inbox placement for Microsoft recipients.
The unique nature of Microsoft's filtering
One of the primary differentiators for Microsoft is their extensive reliance on internal user engagement data and proprietary feedback loops. While most mailbox providers track user interactions, Microsoft's algorithms appear to give these signals an exceptionally high weight. This means even if your IP is technically warmed in terms of volume, a lack of positive user engagement or a high rate of negative interactions can quickly lead to filtering or even a blocklist (or blacklist) designation.
Microsoft's Smart Network Data Services (SNDS) provides a window into your IP reputation, but it's important to understand its limitations. Even if your IP shows a green status in SNDS, indicating general health, your emails might still land in the junk folder. This is because SNDS primarily reflects the overall health of your IP and domain, but doesn't necessarily dictate granular inbox placement decisions, which are influenced by a broader array of real-time user metrics.
Understanding SNDS signals
While SNDS is a useful tool, its green status doesn't guarantee inbox delivery. It mainly provides data on block events, spam complaints, and overall IP reputation, but internal algorithms continuously evaluate sender behavior based on user engagement. Learn more about Microsoft's warm-up process for marketing senders.
Domain reputation: Microsoft heavily weighs the reputation of your sending domain, often even more than the IP. A strong domain reputation, built through consistent positive sending, is paramount. This can even influence why new IPs appear red in SNDS.
User engagement: Opens, clicks, replies, and emails moved from junk to inbox are strong positive signals. Conversely, deletions without opening, marking as junk, or moving to junk are strong negative signals. These are crucial for building sender reputation.
Unlike some providers that might give a new, clean IP a certain grace period, Microsoft's systems are quick to react to perceived negative signals. This means any misstep during the warm-up, such as sending to a high percentage of inactive users or triggering spam complaints, can have an immediate and severe impact on your deliverability. This can lead to your IP being blocked by Microsoft, even after a warm-up.
Adapting your warming strategy for Outlook
Given Microsoft's sensitivity, a standard, aggressive warm-up schedule of increasing volume by 50-100% daily or every other day may be too fast. Instead, a more conservative approach is often necessary. This means slower, steadier increases in volume, sometimes as little as 20-25% every other day, especially in the initial stages. The goal is to consistently demonstrate positive sending behavior without triggering their filters.
The quality of your list during warm-up is paramount for Microsoft. Always prioritize your most engaged subscribers who are highly likely to open and click your emails. These positive interactions send strong signals to Microsoft that your mail is wanted. Avoid sending to any unengaged or old segments, as this can quickly lead to negative reputation scores and filtering.
Content is also a major factor. Ensure your email content is highly relevant, personalized, and avoids common spam triggers, even during warm-up. Consistency in content type and sending patterns helps build a predictable and trustworthy sender profile. Any sudden changes can be flagged as suspicious.
General IP warming
Most email providers prioritize gradual volume increases and basic authentication for IP warm-up success.
Volume increases: Often a 50-100% daily or every-other-day increase.
Focus: Building initial IP reputation through consistent sending patterns.
Monitoring: Primarily looking at bounce rates, spam complaints, and blacklisting.
Microsoft IP warming
For Outlook and Hotmail, the process demands more nuanced engagement and adherence to their specific policies. This is outlined in the best strategy for Microsoft.
Volume increases: Much slower, sometimes 20-25% every other day.
Focus: High positive user engagement and strong domain reputation.
Monitoring: SNDS data, internal filtering, and recipient feedback.
It's also important to maintain consistent sending after the initial warm-up period. Unlike some other providers, Microsoft tends to penalize IPs that have significant gaps in sending volume, as this can signal a potential change in sending behavior that requires re-evaluation. A steady flow of well-received email is key to maintaining trust.
Beyond warm-up: Ongoing reputation management
Successfully warming an IP for Microsoft domains isn't a one-time event, but rather an ongoing process of reputation management. Regularly monitoring your deliverability, even when SNDS shows a green status, is crucial. Keep a close eye on your engagement metrics (opens, clicks) for Microsoft recipients. If these drop, it could indicate a filtering issue even without hard bounces or complaints.
Microsoft, like other providers, places significant emphasis on standard email authentication protocols like SPF, DKIM, and DMARC. Ensuring these are correctly configured and aligned is a foundational step, but it's particularly vital for Microsoft due to their strict adherence to these standards. Poor authentication can quickly lead to emails being marked as junk or outright blocked.
If you do encounter issues, such as your IP being added to an internal Microsoft blocklist, prompt action is required. You may need to submit a delisting request through their support channels, providing details about your sending practices and efforts to resolve any underlying issues. Keep in mind that reputation travel can occur to parent domains.
It's also important to manage your list hygiene meticulously. Hard bounces, spam traps, and high complaint rates can significantly damage your IP and domain reputation, leading to inclusion on a private or public blocklist. Regularly cleaning your list and promptly processing opt-outs can help prevent these issues and maintain the trust that Microsoft demands from senders. This applies to persistent IP reputation issues.
Conclusion
Warming up IP addresses for Microsoft domains requires a more nuanced and patient approach than with many other providers. Their systems are highly attuned to user engagement and proprietary internal signals, placing a greater emphasis on consistent positive interactions and strong domain reputation. By adapting your warm-up schedule, prioritizing highly engaged subscribers, maintaining content relevance, and diligently monitoring your sender reputation, you can navigate Microsoft's unique filtering landscape more effectively and achieve better inbox placement.
Views from the trenches
Best practices
Start with a very small volume, such as 500 messages per IP on the first day, and only increase incrementally.
Prioritize sending to your most engaged contacts during warm-up to generate strong positive engagement signals.
If deliverability stalls or declines, reduce your sending volume and hold back until performance rebounds.
Consistently monitor your engagement metrics and SNDS data to identify potential issues early.
When encountering issues, submit a mitigation request to Microsoft, clearly stating that the IP is new.
Common pitfalls
Increasing sending volume too quickly (e.g., 75% daily increments) can trigger Microsoft's stricter filters.
Sending to unengaged or old segments during warm-up can quickly lead to negative reputation.
Assuming a green status in SNDS guarantees inbox placement, as internal user engagement signals are key.
Ignoring content relevance or quality during warm-up, as this impacts user interaction and reputation.
Failing to maintain consistent sending volume after warm-up, as gaps can negatively impact reputation.
Expert tips
Consider incrementing your sending volume every other day, not daily, for Microsoft domains.
Aim for very conservative volume increases, perhaps around 20-25%, especially in initial stages.
Ensure your list acquisition practices are clean and recipients are actively consenting to receive emails.
Recognize that Microsoft's challenges with IP warming are widespread among senders.
Understand that content relevance and recipient expectations are as crucial as sending volume.
Marketer view
Marketer from Email Geeks says it is challenging to assess the actual audience without more information. Volume is not the sole factor determining reputation; if the same list or acquisition source consistently yields similar results, then the issue likely lies with how addresses are obtained or how content aligns with recipients' expectations. Low opens, even without bounces or spam complaints, can indicate an audience problem.
2019-10-23 - Email Geeks
Marketer view
Marketer from Email Geeks says the audience should not be an issue because the sender is a retailer with clean list acquisition practices. The audience consists of contacts who have explicitly opted into the email program, and engagement, such as active clicking and ordering through emails, is typically high. The content has also remained consistent and previously yielded good results elsewhere.
Microsoft is their extensive reliance on internal user engagement data and proprietary feedback loops. While most mailbox providers track user interactions, Microsoft's algorithms appear to give these signals an exceptionally high weight. This means even if your IP is technically warmed in terms of volume, a lack of positive user engagement or a high rate of negative interactions can quickly lead to filtering or even a blocklist (or blacklist) designation.
Outlook and Hotmail, the process demands more nuanced engagement and adherence to their specific policies. This is outlined in the best strategy for Microsoft.
