Why are emails from Gmail SMTP particularly affected by Microsoft's spam filters?

Emails sent via Gmail SMTP often use shared IP addresses. If other senders on these shared IPs engage in spammy behavior, it can negatively impact the reputation of the entire IP range, leading Microsoft to filter emails more aggressively from those IPs.

My SPF, DKIM, and DMARC are all passing. Why are my emails still going to spam?

While essential, passing authentication records doesn't guarantee inbox placement. Sender reputation, email content, recipient engagement (or lack thereof), and spam complaints play significant roles. Microsoft's filters are also very sensitive to the overall quality and legitimacy of the sending IP's history.

What is the best way to improve deliverability to Outlook, Hotmail, and Live?

The most effective steps include using a reputable third-party SMTP service with dedicated or highly managed shared IPs, maintaining impeccable list hygiene, sending relevant and engaging content, and consistently monitoring your sender and domain reputation .

Does email content affect whether my emails land in spam?

Yes, absolutely. Content that resembles typical spam, such as excessive links, certain keywords, poor formatting, or a high image-to-text ratio, can trigger spam filters regardless of authentication. Personalization and relevance are key.

Knowledge base

/

Email deliverability

/

Troubleshooting

Why are my emails sent from Gmail SMTP ending up in spam folders in Outlook, Hotmail and Live?

Michael Ko

Co-founder & CEO, Suped

Published 15 Jul 2025

Updated 15 Aug 2025

6 min read

It can be incredibly frustrating when you send important emails, especially transactional or outreach messages, and they consistently land in spam folders for recipients using

Outlook, Hotmail, or Live. Even when all your email authentication records like SPF, DKIM, and DMARC appear perfectly configured, Microsoft's filtering can be particularly aggressive. This situation is common, especially when using Gmail's (Google Workspace) SMTP servers for sending.

The core of the problem often lies in how Microsoft's mail servers perceive emails originating from Gmail's shared IP space, combined with factors like sender reputation, content, and recipient engagement. While Gmail is excellent for personal and small-volume business correspondence, its shared SMTP infrastructure can present unique deliverability challenges when sending to Microsoft-hosted inboxes.

Understanding the challenge with Gmail SMTP

One primary reason for deliverability issues from Gmail SMTP to Microsoft properties is the nature of shared IP addresses. When you send emails through Gmail or Google Workspace, you're using IPs shared by millions of other users. If even a small percentage of these users engage in spammy practices, it can negatively affect the collective reputation of that IP, leading to your legitimate emails being flagged.

Microsoft's spam filters are sophisticated and highly sensitive to patterns associated with bulk or unsolicited mail. Even if your emails are personalized and solicited, if they originate from an IP with a questionable history or show characteristics that resemble typical spam (e.g., promotional content, certain formatting), they might be treated as such. Microsoft's systems prioritize the recipient's experience, so they err on the side of caution.

Furthermore, Microsoft (including

Outlook.com, Hotmail, and Live) employs its own internal blacklists (or blocklists) and reputation systems that are not always transparent. Unlike Gmail, which provides Postmaster Tools to give senders insights into their deliverability, Microsoft's feedback loops are less direct, making troubleshooting more challenging. Understanding why deliverability issues with Microsoft Outlook and Hotmail persist is crucial.

Beyond authentication: Sender reputation and content

While SPF, DKIM, and DMARC are foundational, passing these checks simply means your email is technically legitimate. It doesn't guarantee inbox placement. Your sender reputation is paramount. This reputation is built over time based on numerous factors, including spam complaint rates, bounce rates, recipient engagement (opens, clicks), and whether your domain or IP appears on any email blacklists (or blocklists).

Content is another major filter trigger. Even if you're sending solicited emails, certain characteristics can make them look suspicious to automated filters. This includes using excessive links, all-caps, common spam phrases, or a high image-to-text ratio. Misconfigured HTML or unusual character sets can also contribute to emails being flagged as junk. It's often why emails go to the spam folder (and how to avoid that).

Common sender pitfalls

High complaint rates: Recipients marking your emails as spam trains filters to block future messages.
Low engagement: If users rarely open or click your emails, it signals a lack of interest.
Sending to unverified lists: Can lead to hitting spam traps, which severely damages reputation.

Microsoft's perspective

Aggressive filters: Microsoft is known for strict filtering, especially for bulk senders.
Shared IP sensitivity: Poor sending practices from others on Gmail's shared IPs affect you.
Content analysis: Emails resembling marketing or bulk mail are scrutinized more heavily.

Technical considerations and solutions

Even with correct SPF and DKIM records, issues can arise with DMARC alignment, especially when an email passes through multiple mail transfer agents (MTAs) or uses a third-party sending service like HubSpot integrated with Gmail. This can sometimes lead to emails failing DMARC checks from Microsoft's perspective, even if they pass for other providers. You might see warnings like DMARC verification failed in your reports.

One effective solution is to use a dedicated third-party SMTP service (or a reputable ESP) for your bulk or transactional emails instead of Gmail's SMTP. These services often provide dedicated IPs or managed shared IPs with strict sending policies, helping maintain a better sender reputation. This can significantly improve deliverability to Hotmail and other Microsoft domains.

If you must continue using Gmail SMTP for certain types of mail, focus on optimizing content for deliverability. Avoid common spam triggers, use clear and concise subject lines, and ensure your emails are highly relevant to the recipient. Always prioritize sending to engaged subscribers who have explicitly opted in.

Key technical checks

Review DMARC reports: Look for DMARC failures, especially for emails sent to Microsoft domains. If you're not getting reports, ensure your DMARC record is correctly set up.
Check email headers: Analyze headers of emails delivered to spam for insights into why they were flagged.
Monitor blocklists: Regularly check if your domain or sending IP is listed on any major email blocklists (or blacklists). Being listed can severely impact deliverability.

Sustaining good deliverability

Maintaining a healthy sender reputation requires ongoing effort. This includes segmenting your audience and tailoring your email content to their specific interests. Sending relevant, timely, and expected emails reduces the likelihood of spam complaints and boosts engagement metrics.

Regularly cleaning your email list to remove inactive subscribers and invalid addresses is crucial. This not only improves your engagement rates but also reduces bounce rates and minimizes the risk of hitting spam traps. A clean list signals to ISPs (Internet Service Providers) like Microsoft that you are a responsible sender.

Finally, provide an easy unsubscribe option in all your emails. While it might seem counterintuitive, making it simple for recipients to opt out reduces the chance they'll mark your email as spam. A direct unsubscribe is always preferable to a spam complaint in the eyes of mailbox providers.

Views from the trenches

Best practices

Segment your email lists and personalize content to increase engagement.

Regularly clean your email lists to remove inactive or invalid addresses.

Ensure a clear and easy unsubscribe mechanism to reduce spam complaints.

Common pitfalls

Assuming good SPF, DKIM, and DMARC means automatic inbox placement.

Underestimating Microsoft's stricter filtering on shared IP spaces.

Failing to monitor engagement metrics for different mailbox providers.

Expert tips

Actively manage your sending domain's reputation with all mailbox providers, not just Google.

Consider a separate sending infrastructure for marketing or bulk transactional emails.

Use a postmaster tool, like

Marketer view

Marketer from Email Geeks says they often face issues getting emails to Microsoft recipients even with small audiences and solicited mail, pointing to Microsoft's aggressive filtering.

March 10, 2020 - Email Geeks

Expert view

Expert from Email Geeks says that many filters consider even transactional mail as 'bulk' if it's templated and sent to many people, regardless of personalization levels.

March 11, 2020 - Email Geeks

Achieving consistent inbox placement

Getting your emails from Gmail SMTP into Outlook, Hotmail, and Live inboxes is not just about technical setup, but a holistic approach to email deliverability. It involves careful consideration of your sending infrastructure, diligent management of sender reputation, and a commitment to sending highly relevant and engaging content to an opted-in audience.

By understanding Microsoft's stringent filtering mechanisms and proactively addressing the factors that influence deliverability, you can significantly improve your chances of reaching the inbox. Remember that email deliverability is an ongoing process that requires continuous monitoring and adaptation.