How to resolve email deliverability issues to Outlook spam folder after subdomain change?
Michael Ko
Co-founder & CEO, Suped
Published 28 May 2025
Updated 17 Aug 2025
8 min read
Moving to a new subdomain for email sending can feel like a smart strategic move, aiming to segment your email streams or manage sender reputation more effectively. However, it often comes with an unexpected challenge: suddenly, your emails are landing in the spam folder, particularly for major inbox providers like Outlook.com and Live.com. I've seen many email programs face this exact issue, even when they've diligently followed standard best practices for email hygiene and content.
For example, I recently encountered a situation where a company, ExperienceTravel, standardized their sender addresses from various sublabels (like culture@email.experiencetravel.nl) to new, consistent subdomain-based senders (e.g., hello@sublabel.experiencetravel.nl). Despite proper domain warmup, sending only to active users, avoiding spammy content, and ensuring good unsubscribe options, their open rates plummeted, and emails started consistently landing in the spam folder for Outlook and Live addresses.
This scenario highlights a common pain point. Even with a 97% IP reputation, if emails aren't reaching the inbox, something fundamental is amiss with how the new subdomain is being perceived. In this article, I will walk you through the key areas to investigate and the steps I recommend to resolve email deliverability issues to the Outlook spam folder after a subdomain change.
Understanding subdomain impact on deliverability
When you introduce a new subdomain for email sending, you are effectively creating a new sending identity. While it shares the same root domain, email service providers (ESPs) and mailbox providers like Microsoft treat the subdomain as a distinct entity with its own reputation. This means the stellar reputation of your main domain doesn't automatically transfer to the new subdomain. It starts with a clean slate, and any missteps can quickly lead to blocklisting or being routed to the spam (or junk) folder.
This independent reputation is precisely why emails might suddenly go to spam. If the new subdomain hasn't undergone a proper warmup process, or if the initial sending volume from it is too high, it can trigger spam filters. Mailbox providers see sudden, unestablished sending patterns as suspicious, regardless of your past sending history from the root domain. It’s a similar concept to warming up a new IP address, but applied to the domain level.
The good news is that by isolating different email streams (e.g., marketing vs. transactional) onto separate subdomains, you can protect your primary domain's reputation. If your marketing emails encounter deliverability issues, they won't necessarily drag down the deliverability of your critical transactional emails sent from another subdomain. However, this only works if each subdomain is properly managed and its reputation nurtured.
Verify email authentication records
The cornerstone of email deliverability, especially after a subdomain change, is proper email authentication. This includes SPF, DKIM, and DMARC. It's common to overlook configuring these records for a new subdomain, assuming they inherit from the main domain. This is often incorrect, leading to authentication failures that are a major red flag for spam filters, including Outlook's.
For instance, if your SPF record doesn't explicitly authorize your sending service for the new subdomain, or if your DKIM signature isn't correctly set up for it, mail servers will question the legitimacy of your emails. Microsoft (Outlook) is particularly strict, often assigning a high Spam Confidence Level (SCL) to messages failing these checks, which almost guarantees they will land in the junk folder.
Beyond SPF and DKIM, implementing a DMARC policy for your new subdomain is crucial. DMARC tells receiving mail servers what to do with emails that fail SPF or DKIM alignment, and it provides valuable reports that show you which emails are failing and why. If your DMARC monitoring isn't set up to receive reports for your new subdomain, you're flying blind, missing critical insights into your deliverability issues.
Checking DMARC reports for subdomain issues
Accessing DMARC aggregate reports is essential. These XML reports provide a summary of all email traffic originating from your domain (and its subdomains), including authentication results. Look for any instances where your new subdomain is showing SPF or DKIM failures, or where emails are being rejected or quarantined by recipients, particularly Live.com and Outlook.com. This data will directly inform your troubleshooting efforts.
Re-establishing sender reputation
Even with perfect authentication, a new subdomain needs to build its sender reputation from scratch. This process is known as domain warming or IP warming, and it's essential for any new sending identity. It involves gradually increasing your sending volume over time, sending to your most engaged subscribers first. If you push high volumes immediately after a subdomain change, it looks like a sudden, unvetted sender, triggering spam filters.
Key factors for building a good reputation include maintaining low bounce rates, ensuring high engagement (opens and clicks), and minimizing spam complaints. Microsoft's algorithms are sophisticated; they analyze recipient engagement metrics closely. If your emails from the new subdomain aren't being opened or clicked, or worse, if they're being marked as junk, your reputation will suffer, leading to continued spam folder placement.
Proactive engagement is crucial during this phase. Encourage your most loyal subscribers to add your new sending address to their safe sender list or contacts. Also, ask them to move emails from the spam folder to the inbox. This positive feedback signal can significantly boost your new subdomain's reputation with mailbox providers.
Before subdomain change
Plan warmup strategy: Define a gradual increase in sending volume for the new subdomain.
Segment your list: Identify your most engaged subscribers for initial sends to build positive signals.
Inform subscribers: Notify them of the upcoming change to minimize confusion and spam reports.
After subdomain change
Monitor deliverability metrics: Closely track open rates, click-through rates, and spam complaints.
Engage Microsoft support: If issues persist, contact them with details about your subdomain migration.
Encourage positive engagement: Ask recipients to move emails from junk to inbox.
Advanced troubleshooting for Outlook
Beyond the standard authentication and warmup procedures, troubleshooting deliverability to Outlook (and Hotmail/Live) often requires a more targeted approach. Microsoft has specific filtering criteria, and their systems are designed to aggressively filter out potential spam. It's not just about passing SPF, DKIM, and DMARC, but also about how your overall sending behavior and content are perceived.
One area to scrutinize is your email content. Even if you're avoiding obvious spam triggers, subtle elements can still cause issues. This includes the text-to-image ratio, the use of excessive links, or mismatched URLs within the email body compared to your sending domain. Always ensure your content is clean, relevant, and designed for optimal engagement. You can refer to Outlook's new requirements for high-volume senders.
If you've checked all the technical configurations and content, and emails are still consistently going to spam, I recommend directly contacting Microsoft Postmaster support. Provide them with precise details: your new subdomain, the IP addresses used for sending, and clearly state that you are experiencing deliverability issues to Outlook (or Live.com) after a subdomain change and are seeking preemptive mitigation for warmup. Be persistent, as it sometimes requires follow-ups and a clear, concise articulation of your problem to get a resolution. The official support portal is https://olcsupport.office.com/.
Start with a very conservative sending volume for your new subdomain, gradually increasing it over time.
Ensure SPF, DKIM, and DMARC are perfectly configured for the new subdomain, not just the root domain.
Focus on sending highly engaging content to your most active subscribers during the initial warmup period.
Monitor your DMARC reports closely for authentication failures on the new subdomain, addressing them promptly.
Common pitfalls
Assuming existing domain reputation or authentication automatically applies to the new subdomain.
Sending large email volumes from a new subdomain without a proper, gradual warmup.
Neglecting to monitor DMARC reports for authentication and deliverability issues specific to the new subdomain.
Overlooking subtle content issues that might trigger spam filters, even if not overtly spammy.
Expert tips
Be persistent but patient when communicating with Microsoft Postmaster support, providing clear, concise information.
Leverage positive engagement signals by asking recipients to move emails from junk to inbox to boost reputation.
Consider a phased rollout of the new subdomain across your different email programs.
Regularly check your subdomain against common blocklists (blacklists) as a preventive measure.
Expert view
Expert from Email Geeks says that sending a message to aboutmy.email can help diagnose the setup and identify potential DNS issues, even if manual lookups seem correct.
2024-08-26 - Email Geeks
Marketer view
Marketer from Email Geeks says that a sudden drop in open rates and clicks, especially from Outlook and Live addresses, is a strong indicator of deliverability problems.
2024-08-26 - Email Geeks
Moving emails to the inbox
Resolving email deliverability issues to the Outlook spam folder after a subdomain change requires a systematic and patient approach. It's not just about technical configurations, but also about building trust and reputation with mailbox providers. By meticulously verifying your authentication, carefully warming up your new subdomain, continuously monitoring your performance, and being proactive with engagement and support, you can overcome these challenges.
Remember that a subdomain change isn't merely cosmetic, it's a fundamental shift in your email identity. Treating it as such and following the best practices for new senders will significantly improve your chances of consistently reaching the inbox and avoiding the junk folder.
Microsoft treat the subdomain as a distinct entity with its own reputation. This means the stellar reputation of your main domain doesn't automatically transfer to the new subdomain. It starts with a clean slate, and any missteps can quickly lead to blocklisting or being routed to the spam (or junk) folder.
Outlook) is particularly strict, often assigning a high Spam Confidence Level (SCL) to messages failing these checks, which almost guarantees they will land in the junk folder.
Live.com and 
