What to do when Microsoft marks emails as spam from a shared IP address?
Michael Ko
Co-founder & CEO, Suped
Published 29 Jun 2025
Updated 16 Aug 2025
7 min read
Dealing with Microsoft marking your emails as spam can be a frustrating experience, especially when you're sending from a shared IP address. It often feels like you're being penalized for the actions of others on the same IP, even if your own sending practices are impeccable. The core issue is that IP reputation is a significant factor in deliverability, and on a shared IP, that reputation is influenced by all senders, not just you.
When Microsoft (or any major mailbox provider) sees suspicious activity, such as high complaint rates or spam trap hits, originating from a shared IP, the entire IP range can be impacted. This means even legitimate emails from your domain might land in the junk folder, leading to missed password resets, delayed notifications, and frustrated recipients. The challenge is navigating this complex landscape without the luxury of a dedicated IP.
Understanding shared IP challenges
Shared IP addresses present a unique set of deliverability challenges. Unlike a dedicated IP, where your sending reputation is solely your responsibility, a shared IP means you're part of a larger neighborhood. If one sender on that IP behaves poorly, it can negatively impact everyone else's deliverability, including yours. Microsoft's sophisticated spam filters are designed to protect their users, and they don't always distinguish between individual senders on a shared IP as granularly as one might hope.
This collective reputation is what often leads to legitimate emails from shared IPs being routed to the spam folder. Factors like sudden volume spikes, a high number of unknown users, or elevated complaint rates from other senders on the same IP can trigger Microsoft's defenses. It's a tricky situation because even if your list is clean and your content is compliant, you're still susceptible to the actions of others.
While a shared IP can be a cost-effective solution for lower sending volumes, it necessitates heightened vigilance regarding your own sending hygiene. You need to compensate for the shared reputation risk by ensuring your own practices are exemplary and that you are doing everything possible to establish and maintain trust with mailbox providers like Microsoft. If you're encountering persistent issues, you might need to troubleshoot why your emails to Microsoft domains are blocked.
Key technical configurations
Even on a shared IP, proper email authentication is paramount. This includes Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting & Conformance (DMARC). While your sending platform manages the IP, you are responsible for ensuring your domain's authentication records are correctly configured and aligned.
A common pitfall on shared IPs is failing to set up DKIM to sign with your own domain, or that your DKIM signature is temporary. Many providers offer a default DKIM signature that uses their domain, not yours. You should always configure DKIM to sign with your domain (d=yourdomain.com) to establish your own sender reputation, distinct from the shared IP's reputation. This provides a crucial layer of trust.
Beyond DKIM, ensure your SPF record authorizes your sending provider's IPs and that you have a robust DMARC policy. These protocols work together to verify that your emails are legitimate and haven't been tampered with. A strong authentication setup signals to Microsoft that you are a trustworthy sender, even when sharing an IP.
Always prioritize setting up proper email authentication records. While SPF and DMARC are crucial, pay special attention to ensuring your DKIM signature uses your own domain, not that of your email service provider. This directly impacts your domain's sending reputation with Microsoft.
Engaging with Microsoft and your ESP
When facing deliverability issues, contacting Microsoft support can be a necessary step, especially through their Anti-Spam IP Delist Portal. However, with a shared IP, the process can be less straightforward. They often require the specific IP address, and if it's shared, they may be reluctant to make individual adjustments without seeing broader issues from the service provider.
Your Email Service Provider (ESP), like Iterable or AWS SES, is your primary point of contact for shared IP issues. They manage the IP infrastructure and are usually registered with services like Microsoft's Sender Network Data Services (SNDS). Inquire if they can assign you to a better-performing shared IP, if possible, or assist with delisting requests directly with Microsoft. They have the most leverage to resolve broader IP reputation problems.
Remember that while you can report issues, the ultimate control over a shared IP's reputation lies with the service provider managing it. Open communication with your ESP about your deliverability concerns to Microsoft is crucial. This proactive approach can help them address underlying issues that might be affecting the entire shared pool.
Shared IP challenges
Reputation vulnerability: Your deliverability is affected by other senders' practices.
Limited control: Direct delisting requests to Microsoft for a shared IP are often ineffective.
Provider dependency: You rely on your ESP to manage the overall IP reputation.
Dedicated IP advantages
Full control: Your sending practices directly dictate your IP reputation.
Direct recourse: You can directly manage delisting requests if your IP is on a blacklist.
IP warming: You control the process to build a new IP's reputation.
Improving your sender reputation
Even with shared IPs, nurturing a strong sender reputation is key to improving email deliverability with Microsoft. Your domain's reputation, driven by recipient engagement and compliance, plays a critical role. Focus on sending wanted mail, maintaining a clean list, and minimizing negative signals.
High engagement rates (opens, clicks, replies) signal to Microsoft that your emails are valuable to recipients. Conversely, high bounce rates, low engagement, and especially spam complaints, can quickly damage your domain's standing, even if the IP's reputation is influenced by others. Actively encourage recipients to move your emails from junk to inbox and to add you to their safe sender list.
Regularly clean your email lists to remove inactive or invalid addresses, which can become spam traps. Implement clear unsubscribe options and honor requests promptly. While you can't control the actions of other senders on your shared IP, you can absolutely control your own sending practices to bolster your domain's reputation. This is critical for fixing emails going to spam.
Factor
Impact on deliverability
Action to take
Engagement rate
High opens/clicks signal good content; low engagement can hurt.
Directly damages reputation and can lead to blacklisting.
Implement easy unsubscribe, send only to opt-in lists.
Bounce rates
High rates indicate poor list hygiene and can flag you as a spammer.
Regularly clean lists, use double opt-in.
Authentication
Proper SPF, DKIM, and DMARC build trust with Microsoft.
Ensure all records are correctly configured and aligned.
Views from the trenches
Best practices
Ensure your DKIM signing is explicitly configured for your own domain, not your provider’s default.
Maintain high engagement rates by sending relevant content to an interested audience.
Proactively clean your email lists to minimize bounces and spam trap hits.
Common pitfalls
Relying solely on your ESP's default DKIM setup, which may use their domain, not yours.
Neglecting list hygiene, leading to sending to inactive or invalid addresses on a shared IP.
Expecting Microsoft to delist a shared IP without sufficient evidence of good sending practices.
Expert tips
Consider segmenting your audience and sending critical transactional emails from a potentially cleaner pool if your ESP offers this flexibility.
Educate your recipients on how to mark your emails as 'not spam' or add you to their safe sender list.
For very low volumes on a shared IP, even a few complaints can have a disproportionately large impact, so aim for zero complaints.
Marketer view
Marketer from Email Geeks says they can submit the Microsoft delist form even for shared IPs, but it's often not very effective without proper DKIM setup for their domain.
2022-11-18 - Email Geeks
Marketer view
Marketer from Email Geeks says that many on shared IPs don't complete DKIM setup, resulting in a provider-issued signature, which is detrimental to their own reputation.
2022-11-18 - Email Geeks
Moving forward
While a shared IP can present unique deliverability hurdles with Microsoft, it's not an insurmountable obstacle. The key is to optimize the elements you can control: your email authentication, sender reputation, and direct communication with your ESP. By focusing on these areas, you can significantly improve your chances of reaching the inbox, even when sharing IP space with others.
Remember that consistent, positive sending behavior is the most powerful tool in your deliverability arsenal. Combine strong technical configurations with excellent list hygiene and engaging content, and you'll be well on your way to earning Microsoft's trust for your email program.
Microsoft. If you're encountering persistent issues, you might need to troubleshoot why your emails to Microsoft domains are blocked.
Iterable or 
AWS SES, is your primary point of contact for shared IP issues. They manage the IP infrastructure and are usually registered with services like Microsoft's Sender Network Data Services (SNDS). Inquire if they can assign you to a better-performing shared IP, if possible, or assist with delisting requests directly with Microsoft. They have the most leverage to resolve broader IP reputation problems.
