Why are emails from Gmail or Gsuite accounts landing in spam when replying in threads with Microsoft accounts?
Michael Ko
Co-founder & CEO, Suped
Published 12 May 2025
Updated 16 Aug 2025
8 min read
It's a frustrating scenario many businesses and individuals face: emails from Gmail or Gsuite accounts landing in spam when you're replying in ongoing threads with Microsoft accounts. You'd think that replying within an existing conversation would signal legitimacy to spam filters, but often, that's not the case. This specific issue highlights the complex and often unforgiving nature of email deliverability, especially when dealing with different email providers like Gmail and Microsoft Outlook.
This problem isn't always about a single culprit; it's often a combination of factors related to email authentication, sender reputation, content, and the specific filtering behaviors of receiving mail servers. Understanding these elements is key to diagnosing and resolving why your legitimate replies are being flagged as spam.
Let's explore the common reasons behind this challenge and what steps you can take to improve your email deliverability when communicating across Gmail or Gsuite to Microsoft accounts.
The role of email authentication
Email authentication is the bedrock of deliverability, even for personal emails or replies in existing threads. SPF, DKIM, and DMARC records prove that your emails are legitimately coming from your domain and haven't been tampered with. Microsoft, like other major providers, heavily relies on these protocols to filter out spam and phishing attempts. A misconfigured or missing record can immediately flag your emails, regardless of the conversation history.
Even if your initial email was delivered successfully, subsequent replies can still encounter issues if authentication isn't robust. For instance, if your DMARC policy is set too strictly and there are minor authentication glitches in a reply, Microsoft's filters might still view it with suspicion. Ensuring all three authentication mechanisms are correctly set up and aligned is crucial. You can learn more about these by checking out a simple guide to DMARC, SPF, and DKIM.
For Gmail and Gsuite accounts, proper domain authentication is the biggest single thing you can do to avoid spam folders. While Gmail typically handles the technical setup for Gsuite users, it's still your responsibility to ensure your DNS records are correctly configured on your domain host. Without a proper SPF or DKIM authentication, your emails are at a significant disadvantage.
Microsoft's email filtering is notoriously aggressive. While Gmail is known for its advanced spam detection, Microsoft's filters can sometimes be less accurate, especially when dealing with high volumes of legitimate mail or domains that are also used by spammers. This means that even if your Gmail or Gsuite account has a good reputation with Google, it might face different challenges with Microsoft's systems.
Microsoft's spam filters (or blocklists) are known to be particularly sensitive to sudden changes in sending patterns or even specific content characteristics, even within established threads. If your Gmail or Gsuite domain has any lingering negative reputation, it can disproportionately affect deliverability to Microsoft inboxes. This could be due to past spam complaints, low engagement, or even shared IP addresses that have a poor reputation. This is why emails often land in Office 365 spam folders.
For more information on Microsoft'sspam flagging criteria, you can consult their support resources. It's a known issue that Gmail (and Google Workspace) has become a significant source of spam, which could contribute to the heightened scrutiny from Microsoft's filters, making Gmail or Gsuite emails more prone to landing in spam.
Gmail/Google Workspace Filtering
Google's filters are highly sophisticated, using AI and machine learning to analyze content, sender behavior, and historical data. They prioritize user experience by blocking 99.9% of spam and malware.
They often provide granular feedback through Google Postmaster Tools, allowing senders to monitor their domain reputation and spam rates. The emphasis is on proactive prevention and transparent data for senders.
Microsoft Outlook Filtering
Microsoft's filters are known for being very aggressive, sometimes flagging legitimate emails as spam. They can be less transparent about their filtering logic, making it harder for senders to diagnose issues.
They are particularly sensitive to shared IP addresses (common with Gmail and Gsuite consumer accounts) and content that might trigger their internal rules. Even with good authentication, some emails may struggle with Microsoft due to their stringent, sometimes opaque, filtering criteria.
Content and threading impact
Even when you're replying in an ongoing conversation, the content of your email plays a critical role. Spam filters analyze various elements within your message, including keywords, phrases, link patterns, and overall message structure. If your reply contains elements commonly associated with spam, it can be flagged, even within a legitimate thread.
Common culprits include: excessive use of exclamation marks, all caps, suspicious links (especially in email signatures that might change or be shortened), or attachments that trigger security warnings. Gmail, Outlook, and other webmail providers use a spam filter that detects certain words and phrases.
Even the structure of the reply can matter. While most email clients handle threading well, certain quirks in formatting or the inclusion of unusual characters can sometimes confuse filters. Although it’s impossible to know exactly what will and won't go to spam, being mindful of your email content can significantly help. This also explains why emails are blocked as spam in ongoing conversations.
Content best practices
Review links: Ensure all links in your signature or body are reputable and don't redirect through suspicious services.
Avoid spam triggers: Minimize excessive capitalization, exclamation marks, or phrases commonly found in spam.
Clean formatting: Stick to standard fonts and formatting. Avoid overly complex HTML or unusual characters that might be misinterpreted.
IP and domain blocklisting
One of the most direct reasons for emails landing in spam is when your sending IP address or domain finds itself on an email blocklist (or blacklist). These lists are maintained by various organizations to help email providers filter out known spammers. If a Gmail or Gsuite IP address (especially common ones used by many users) or your domain is blocklisted, Microsoft will likely reject or junk your emails.
Even if Gmail itself isn't on a blocklist, the specific IP address your email routed through might be temporarily listed due to other senders' poor practices. Regularly checking your domain and IP health using a blocklist checker can help you identify and address these issues promptly. Understanding how email blacklists actually work is a crucial step for maintaining good deliverability.
If your domain is blacklisted, it doesn't matter how good your email content or authentication is, your emails will still be blocked. This is particularly relevant because Microsoft is known to check your IP/Domain health using various tools. Prompt action is needed to request delisting if you find yourself on a blocklist. You can monitor this proactively using a blocklist monitoring service.
Blocklist Type
Impact
Example
IP-based blocklists
Block emails from specific IP addresses known for spamming. This can affect Gmail or Gsuite if their shared IPs are listed.
Spamhaus PBL
Domain-based blocklists
Block emails from specific domains that have been associated with spam. Direct impact on your custom Gsuite domain.
SURBL
Real-time blackhole lists (RBLs)
Lists of IP addresses that have been observed sending spam or compromised with malware. Used in real-time by mail servers like Microsoft.
UCEPROTECT
Recent changes and their impact
Email deliverability is a constantly evolving landscape. Microsoft has, in recent times, implemented new sender requirements aimed at reducing spam. These changes often involve stricter adherence to authentication standards and better sender practices. If your Gmail or Gsuite setup isn't fully compliant with these new rules, your emails may struggle.
Furthermore, a recurring theme is that Gmail and Google Workspace are increasingly used by spammers. This can lead to broader reputation issues for IPs associated with Google's services, making all emails originating from these sources subject to higher scrutiny from other providers like Microsoft. This affects both bulk senders and individual users. You can read more about why emails to Gmail experience delivery delays due to similar issues.
Staying informed about these changes and proactively adjusting your email practices is critical. Neglecting new requirements or changes in spam filtering algorithms will inevitably lead to more emails landing in spam folders.
Views from the trenches
Best practices
Maintain strong authentication (SPF, DKIM, DMARC) for your domain, even for personal use.
Regularly monitor your domain and sending IP for blocklist presence.
Keep email content clean, concise, and free of typical spam triggers.
Segment your email lists and send to engaged recipients to maintain good sender reputation.
Encourage recipients to add you to their safe sender list or address book.
Common pitfalls
Ignoring DMARC reports, which provide valuable insights into authentication failures.
Using generic email signatures with multiple links or tracking pixels that could be flagged.
Sending large volumes of emails from a new or unestablished Gmail/Gsuite account.
Not monitoring your sender reputation via Google Postmaster Tools or other services.
Assuming replies in threads are automatically safe from spam filters.
Expert tips
Consider setting a strong DMARC policy (p=quarantine or p=reject) once confident in your authentication setup.
If using a custom domain with Gmail/Gsuite, ensure your DNS records are managed correctly and not just relying on default settings.
Be aware that Microsoft's filters often act unpredictably, sometimes even with perfect setup.
For critical communications, advise recipients to check their junk folder and add you to their safe sender list.
If you're still experiencing issues, analyze message headers for detailed delivery information and specific reasons for filtering.
Marketer view
Marketer from Email Geeks says Microsoft is known to act in a wildcard fashion regarding email acceptance and folder placement, often putting emails into the junk folder even when added as a "safe sender". Email signatures with links can also get caught in Microsoft's Safelinks reputation module.
2023-04-18 - Email Geeks
Marketer view
Marketer from Email Geeks says Gmail and Google Workspace are significant sources of spam these days, which can make Microsoft's filters more aggressive. Many B2B spammers are now using @gmail.com addresses.
2023-04-18 - Email Geeks
Navigating cross-platform deliverability
Dealing with emails from Gmail or Gsuite accounts landing in Microsoft spam folders, even within threads, is a common headache for many. It's rarely one single issue but rather a combination of factors, each contributing to the overall deliverability challenge. Proactive management of your email ecosystem is key.
By ensuring robust authentication, maintaining a clean sender reputation, being mindful of your email content, and regularly monitoring for blocklists, you can significantly improve your chances of reaching the inbox. While Microsoft's filters can be unpredictable, consistent adherence to best practices will always yield the best results for your email deliverability.
Gmail and 
Microsoft Outlook.
Google's filters are highly sophisticated, using AI and machine learning to analyze content, sender behavior, and historical data. They prioritize user experience by blocking 99.9% of spam and malware.
Microsoft's filters are known for being very aggressive, sometimes flagging legitimate emails as spam. They can be less transparent about their filtering logic, making it harder for senders to diagnose issues.
Gmail or Gsuite if their shared IPs are listed.
