Why are emails from Microsoft accounts being rejected by Gmail, and what is the role of IPv6?
Michael Ko
Co-founder & CEO, Suped
Published 22 Jun 2025
Updated 17 Aug 2025
8 min read
For many organizations, email communication is a lifeline. When emails from Microsoft accounts, particularly Outlook or Microsoft 365, start getting rejected by Gmail, it can be a significant disruption. This issue often surfaces with non-delivery reports (NDRs) indicating a "domain reputation issue", leaving senders puzzled since their own domain might have a clean sending history elsewhere.
The core of this problem frequently lies with how Google perceives email traffic originating from Microsoft's IPv6 ranges. It appears that a history of problematic or spammy mail from these IPv6 addresses has led Gmail to adopt a stricter stance, often rejecting legitimate emails as a preventative measure. This isn't necessarily about your specific sending domain, but rather the reputation of the underlying network infrastructure from which the email is sent.
Understanding this dynamic, particularly the role of IPv6, is crucial for anyone relying on Microsoft services for email delivery to Gmail recipients. We'll explore the reasons behind these rejections and what steps can be taken to improve email deliverability.
The core issue: IPv6 reputation challenges
My experience has shown that the "domain reputation issue" error message from Gmail, when receiving mail from Microsoft accounts, often refers not to your sending domain but to the reputation of the sending IP address (or range) itself. Gmail has historically observed a high volume of unsolicited mail (spam) originating from certain IPv6 addresses used by Microsoft. As a result, its systems are configured to heavily scrutinize, and often reject, emails coming from these IPv6 ranges, even if the individual sending domain is legitimate.
This preventative measure is designed to protect Gmail users from spam. Unfortunately, it can inadvertently affect legitimate senders. Google has explicitly stated that emails originating from Outlook's IPv6 addresses were previously marked as spam due to their reputation, leading to ongoing rejections. This highlights a broader challenge in email deliverability: the shared reputation of large email service providers' IP ranges.
While Microsoft and Google have been in dialogue to address these issues, the impact can still be felt by users. It is a constant battle to maintain a good sending reputation when your email infrastructure is shared with many other senders, some of whom may not adhere to best practices. This can lead to a blocklist (or blacklist) designation for parts of a shared IP range, affecting innocent senders.
Technical deep dive into IPv6 and email
When an email is sent over IPv6, Gmail applies more stringent guidelines regarding PTR records and authentication. A common rejection message might look something like this:
Example Gmail Rejection Message
550-5.7.1 Our system has detected that this message does not 550-5.7.1 meet IPv6 sending guidelines regarding PTR records and 550-5.7.1 authentication.
This error points directly to issues with fundamental email authentication mechanisms when sending over IPv6. While SPF, DKIM, and DMARC are critical for all email, their correct configuration is paramount for IPv6. PTR records, also known as pointer records, perform a reverse DNS lookup, mapping an IP address back to a domain name. This helps verify the sender's legitimacy.
The challenge with IPv6 often stems from its sheer size. The vast number of available IPv6 addresses makes it more difficult for mailbox providers to track and assign reputation in the same way they do for IPv4. Spammers can more easily cycle through IPv6 addresses to evade detection, leading providers like Gmail to apply a broad-brush approach by being very strict on the authentication and PTR record requirements for IPv6 traffic, especially from large networks. For more context on why IPv6 emails go to spam, see why IPv6 emails go to spam.
Mitigating the problem
While you don't directly control Microsoft's IPv6 reputation, there are several steps you can take to mitigate the impact of these rejections on your email deliverability. One of the most common workarounds, as suggested by Google itself in some instances, is to ensure your emails are sent over IPv4.
This might involve configuring your email send connector to prefer IPv4 or, if available through your provider, specifically request IPv4 sending. This has proven effective for some organizations experiencing this issue. Additionally, maintaining impeccable email authentication, including SPF, DKIM, and DMARC records, is always essential for demonstrating legitimacy.
Monitoring your sender reputation using tools like Google Postmaster Tools can provide valuable insights into how your domain is perceived. While it might not show specific IPv6 issues directly impacting Microsoft's broader IP ranges, it can help you identify if your own sending practices are contributing to any deliverability problems. Being proactive about list hygiene and engagement is key to avoiding blocklists (or blacklists) and maintaining a strong sending reputation, regardless of the underlying IP version.
If you are experiencing rejections from Gmail due to IPv6 reputation issues, consider these options:
Configure Connectors: Set up email connectors in Microsoft 365 to specifically route traffic to Gmail via IPv4 if possible. Some users have found success forcing this setting to avoid the problematic IPv6 routes.
Review Authentication: Double-check your SPF, DKIM, and DMARC records to ensure they are correctly configured and aligned. Even if the issue is with Microsoft's IP reputation, having strong authentication for your domain is always a prerequisite for good deliverability.
Monitor Deliverability: Keep a close eye on your email deliverability rates and bounce messages. This will help you identify if the issue persists or if new problems arise. For specific Microsoft-related bounces, consult Microsoft email address bouncing.
Contact Support: If the problem persists, reach out to Microsoft support to inquire about any known issues with IPv6 sending to Gmail and if there are specific recommendations or workarounds they can provide for your account.
Moving forward: Microsoft and Gmail's roles
The good news is that both Microsoft and Google are aware of these specific deliverability challenges. Reports from direct communications indicate that engineering teams at both companies have been in touch regarding these issues affecting users. This ongoing dialogue is crucial, as it suggests a commitment to finding a resolution. While a quick fix isn't always available, the acknowledgment from both sides is a positive step toward improving email flow.
Ultimately, the transition to IPv6 for email sending is complex. While IPv6 offers a vast address space, which is critical for the internet's growth, it also presents unique challenges for maintaining sender reputation and combating spam. Google's stricter approach to IPv6 is a reflection of these challenges and its efforts to protect its users.
As the internet evolves, so too will email protocols and deliverability standards. Staying informed about these changes and proactively managing your email sending practices, including robust authentication and reputation monitoring, will be vital for ensuring your messages reach the inbox consistently, whether they're sent over IPv4 or IPv6.
Views from the trenches
Best practices
Always ensure SPF, DKIM, and DMARC are properly configured for your sending domains to bolster legitimacy.
Proactively monitor your email deliverability and any bounce messages for early detection of issues.
Maintain excellent list hygiene to reduce bounces and avoid spam traps, which can harm your sender reputation.
Common pitfalls
Overlooking IPv6-specific sending guidelines and authentication requirements can lead to unexpected rejections.
Assuming that good IPv4 deliverability automatically translates to good IPv6 deliverability is a common mistake.
Failing to address shared IP reputation issues, particularly on large email service provider networks.
Expert tips
If possible, configure email send connectors to prioritize IPv4 for critical communications to Gmail.
Engage with your email service provider's support team to inquire about IPv6 deliverability best practices and known issues.
Regularly check blocklists (or blacklists) to ensure your sending IPs haven't been affected by broader network reputation issues.
Expert view
Expert from Email Geeks says: Sending mail to Gmail over IPv6 can lead to significant problems if not managed correctly, even for large providers like Microsoft.
2024-06-04 - Email Geeks
Expert view
Expert from Email Geeks says: Google imposes stricter standards for email sent over IPv6, and it's not surprising that Microsoft's IPv6 ranges have accumulated a poor reputation due to spam.
2024-06-04 - Email Geeks
Ensuring your emails reach the inbox
Navigating the complexities of email deliverability, especially when major providers like Microsoft and Google are involved, requires diligence and an understanding of underlying network protocols. While IPv6 is the future of the internet, its current implementation in email environments, coupled with historical reputation issues, can lead to unexpected rejections. By understanding the role of IPv6, ensuring proper authentication, and employing strategic workarounds like favoring IPv4, you can significantly improve the success rate of your emails from Microsoft accounts reaching Gmail inboxes.
Outlook or 
Microsoft 365, start getting rejected by 
Gmail, it can be a significant disruption. This issue often surfaces with non-delivery reports (NDRs) indicating a "domain reputation issue", leaving senders puzzled since their own domain might have a clean sending history elsewhere.
Google perceives email traffic originating from Microsoft's IPv6 ranges. It appears that a history of problematic or spammy mail from these IPv6 addresses has led Gmail to adopt a stricter stance, often rejecting legitimate emails as a preventative measure. This isn't necessarily about your specific sending domain, but rather the reputation of the underlying network infrastructure from which the email is sent.
Google itself in some instances, is to ensure your emails are sent over IPv4.
Microsoft 365 to specifically route traffic to Gmail via IPv4 if possible. Some users have found success forcing this setting to avoid the problematic IPv6 routes.
