Why do emails sent through SendGrid go to spam on Outlook.com, but those from O365 do not?
Matthew Whittaker
Co-founder & CTO, Suped
Published 18 Apr 2025
Updated 19 Aug 2025
6 min read
When emails sent through SendGrid land in spam folders on Outlook.com, while those from Office 365 reach the inbox, it can be a perplexing and frustrating problem. This common scenario highlights key differences in how major email service providers (ESPs) and direct corporate mail systems like O365 are perceived by recipient mail servers, particularly Microsoft's stringent filtering. The core of the issue often lies in sender reputation, authentication, and content, but the specifics vary significantly based on the sending infrastructure.
It's tempting to think that all emails from a trusted domain should behave similarly, regardless of the sending platform. However, the underlying infrastructure, shared IP pools versus dedicated ones, and the overall sending practices of an ESP's user base can heavily influence deliverability. Microsoft's anti-spam filters are particularly sensitive to these nuances, often treating emails from high-volume third-party senders differently than those originating directly from a dedicated O365 instance.
Understanding sender reputation and infrastructure
The primary reason for this discrepancy often comes down to sender reputation, especially at the IP level. Office 365 (O365) emails, when sent directly from your corporate account, originate from Microsoft's own highly reputable IP ranges, which benefit from Microsoft's internal trust mechanisms and careful management. These IPs generally have an inherently strong reputation due to their strict control, making them less likely to be flagged as spam by Microsoft's own filters.
Conversely, SendGrid, as a transactional email service provider, operates large networks of shared and dedicated IP addresses. While they strive to maintain high deliverability, the reputation of these IPs can be influenced by the sending practices of all users who share them. Even on a dedicated IP address with SendGrid, your reputation is still tied to the broader ecosystem, and any negative activity from other SendGrid users could potentially impact shared IP performance. This can lead to your emails (even those with good intentions) being caught in blocklists or spam filters. For more details on why some emails fail, see our expert guide on email deliverability issues.
Microsoft's filtering system is known for being aggressive, and it often has a tighter grip on how it evaluates incoming mail, especially from external ESPs. They employ complex algorithms that consider not just authentication, but also sender history, volume, complaint rates, and engagement metrics linked to the sending IP and domain. If an IP or domain has accumulated negative signals from mass email campaigns, even individual emails may suffer.
Authentication and alignment differences
Email authentication protocols like SPF, DKIM, and DMARC play a crucial role in deliverability. While both SendGrid and O365 support these, their implementation and how they contribute to your domain's overall trust can differ. When sending directly from O365, these records are typically configured to perfectly align with Microsoft's sending infrastructure, providing a seamless authentication path.
With SendGrid, you need to ensure your domain's SPF and DKIM records properly authorize SendGrid's servers to send on your behalf. A common issue is misconfiguration, where records might pass syntactically but still fail alignment checks, particularly DMARC alignment. This can cause legitimate emails to fail authentication, leading to them being marked as spam or even silently dropped by recipient servers. Learn more about simple DMARC examples.
You can compare how authentication setups might impact deliverability using a side-by-side view.
O365 direct sending
IP reputation: Benefits from Microsoft's highly trusted internal IP ranges with a strong historical reputation.
Authentication: Naturally aligns with Microsoft's own authentication mechanisms (SPF, DKIM, DMARC), ensuring high trust.
Volume impact: Typically lower volumes for individual accounts, leading to more consistent reputation for personal emails.
Filtering: Less likely to be flagged by Outlook's internal filters due to direct origin from trusted Microsoft infrastructure.
SendGrid sending
IP reputation: Relies on SendGrid's shared or dedicated IP addresses, whose reputation can be influenced by other SendGrid users.
Authentication: Requires careful configuration of SPF and DKIM records to authorize SendGrid, with DMARC alignment being crucial. Sometimes, issues arise even when SPF and DKIM pass, as explained in this Microsoft forum thread.
Volume impact: Often used for high-volume campaigns, which, if not managed correctly, can lead to reputation degradation and blocklisting.
Filtering: More susceptible to Outlook's aggressive filters, especially if sender behavior or content triggers spam signals.
Content and sender behavior
Beyond technical configurations, the actual content of your emails and your sending behavior significantly impact deliverability, especially with Microsoft's filters. O365 personal emails are typically one-to-one communications, often short, and less likely to contain elements common in marketing or bulk mail. This inherently reduces spam signals.
SendGrid emails, even transactional ones, often resemble marketing messages, containing tracking links, images, and specific formatting that can trigger spam filters if not carefully managed. High complaint rates, low engagement (e.g., recipients not opening or clicking), and sending to inactive or invalid addresses (spam traps) can quickly damage your sender reputation, leading to emails being junked.
It is crucial to monitor your sending patterns and recipient engagement. Unsubscribe mechanisms are important for compliance and good sending practice, as noted in a SendGrid article. A high volume of unengaged recipients or spam complaints will send strong negative signals to recipient mail servers. For more information, refer to the "What to do if my Email Messages are Blocked?" article on the SendGrid support site.
Key factors influencing deliverability
Content: Avoid spammy keywords, excessive images, broken links, or misleading subject lines. Personalize emails where possible.
Engagement: Monitor open and click rates. Low engagement signals can lead to emails landing in spam. Remove inactive subscribers.
Spam complaints: Even a small number of complaints can severely impact your reputation. Ensure easy unsubscribe options.
List hygiene: Regularly clean your email lists to remove invalid or unengaged addresses to avoid hitting spam traps.
Mitigating SendGrid deliverability issues with Outlook
If your SendGrid emails are consistently going to spam (or junk) on Outlook.com, there are several steps you can take to improve your deliverability. First, ensure your SPF, DKIM, and DMARC records are correctly set up and align properly. DMARC reporting can help you identify authentication failures and sources of non-compliant mail.
Next, focus on your sending practices. Segment your audience and send relevant content. Implement a proper warm-up strategy for new IPs or domains to gradually build a positive sending reputation. Monitor your email engagement metrics closely and regularly clean your lists to remove unengaged subscribers or spam traps. Microsoft is more likely to trust senders with consistent positive engagement. If you are experiencing deliverability problems, you can review our article on troubleshooting email deliverability issues with Microsoft Outlook and Hotmail.
Finally, consider registering for Microsoft's Smart Network Data Services (SNDS) and Junk Mail Reporting Program (JMRP). These tools provide insights into how Microsoft views your sending reputation and allow you to receive feedback on spam complaints, helping you react quickly to any issues. Proactive monitoring and adherence to best practices are key to successful email delivery.
Example SPF record entry for SendGrid
"v=spf1 include:sendgrid.net ~all"
If you have other sending services, you'll combine them:
"v=spf1 include:sendgrid.net include:other-esp.com ~all"
Views from the trenches
Best practices
Actively monitor your SPF, DKIM, and DMARC records for correct setup and alignment.
Use Microsoft's SNDS and JMRP programs to gain insight into your sending reputation with Outlook.
Implement a gradual IP warm-up process, especially for new dedicated IPs, to build trust.
Segment your email audience to ensure relevant content delivery and higher engagement rates.
Regularly clean your email lists to remove inactive users and avoid spam traps.
Common pitfalls
Assuming all emails from a trusted domain will reach the inbox, regardless of sending platform.
Not having full visibility into your SendGrid IP reputation and complaint rates.
Sending high volumes of emails without proper list segmentation and engagement management.
Ignoring DMARC reports, which provide critical data on authentication failures and policy compliance.
Overlooking the impact of content and email design on spam filter triggers.
Expert tips
Microsoft Outlook's filtering is highly sensitive to IP reputation, even for dedicated IPs.
O365's inherent trust with Outlook.com is due to its 'too big to block' status and consistent low-volume, direct sending.
Even a few spam complaints, if consistent, can severely damage deliverability for a SendGrid IP.
Always verify broad 'all emails are going to spam' claims with data, as they are often exaggerations.
The content and sending patterns for transactional emails sent via ESPs are often scrutinized differently than direct peer-to-peer emails.
Expert view
Expert from Email Geeks says to start by checking all the usual technical configurations like SPF, DKIM, and DMARC. Also, gaining access to SNDS and Google Postmaster Tools for monitoring, and reviewing reports on bounces, spam trap hits, and inbox placement is crucial.
2019-05-15 - Email Geeks
Marketer view
Marketer from Email Geeks says they found their corporate environment sends emails out of Office 365, noting that emails from there go to the inbox, while campaign emails from the same domain via SendGrid go to spam.
2019-05-15 - Email Geeks
A holistic approach to inbox placement
Ultimately, the difference in deliverability between emails sent directly from O365 and those sent through SendGrid on Outlook.com boils down to a combination of IP reputation, meticulous authentication, and sender behavior. While O365 benefits from Microsoft's inherent trust, SendGrid users must actively manage their reputation and practices to achieve optimal inbox placement. Addressing these factors systematically is vital for improving your email deliverability.
Microsoft's anti-spam filters are particularly sensitive to these nuances, often treating emails from high-volume third-party senders differently than those originating directly from a dedicated O365 instance.
Microsoft's own highly reputable IP ranges, which benefit from Microsoft's internal trust mechanisms and careful management. These IPs generally have an inherently strong reputation due to their strict control, making them less likely to be flagged as spam by Microsoft's own filters.
Relies on SendGrid's shared or dedicated IP addresses, whose reputation can be influenced by other SendGrid users.
