Why do emails sent through SendGrid go to spam on Outlook.com, but those from O365 do not?

Emails sent through SendGrid are more likely to land in Outlook.com's spam folder compared to those from Office 365 due to a combination of factors, primarily revolving around trust, reputation, and stringent authentication requirements for third-party senders. Office 365 emails benefit from Microsoft's inherent internal trust, its robust infrastructure, and the high resilience of its IP and domain reputation within its own ecosystem. Conversely, SendGrid, as an external Email Service Provider, must actively build and maintain a strong sender reputation with Microsoft's filters. This involves meticulous attention to technical authentication like SPF, DKIM, and especially DMARC alignment, which is often handled natively for O365. Furthermore, SendGrid users must manage their IP and domain reputation, which can be negatively affected by shared IP pools or insufficient warming of dedicated IPs. The content of the email, recipient engagement metrics (like open rates and complaint rates), and diligent list hygiene also play significant roles. Microsoft's new bulk sender requirements for external senders further emphasize the need for strict adherence to authentication, easy unsubscription, and maintaining very low spam complaint rates, making it more challenging for SendGrid users to consistently reach the inbox without diligent effort.

Emails sent via SendGrid often encounter deliverability challenges, leading them to Outlook.com's spam folder, while those from Office 365 generally reach the inbox. This disparity stems from Microsoft's inherent trust in its own ecosystem. Office 365 leverages its extensive, resilient infrastructure and highly trusted IP ranges, which are less susceptible to blocking and are automatically aligned for authentication protocols. In contrast, SendGrid, as an external sender, must diligently establish and maintain sender trust and a robust reputation with Microsoft's stringent filters. This involves meticulous attention to technical authentication, particularly DMARC alignment, which Outlook.com heavily scrutinizes. Furthermore, SendGrid users, especially those on shared IP pools, can be adversely affected by the poor sending practices of others, and even dedicated IPs require careful, gradual warming to build a solid reputation. The content of the email itself, recipient engagement metrics, and diligent list hygiene are also critical factors influencing Outlook.com's spam filtering decisions, with Microsoft's SmartScreen filter being particularly sensitive to suspicious elements.

Emails sent through SendGrid face increased scrutiny from Outlook.com, often leading to their placement in spam folders, a contrast to the reliable inbox delivery typically seen with emails from Office 365. This disparity arises from Microsoft's highly sensitive, reputation-based filtering and new, stringent bulk sender requirements. As of February 2024, Microsoft mandates strict adherence to email authentication standards (SPF, DKIM, DMARC), easy unsubscription, and exceptionally low spam complaint rates for all bulk senders. SendGrid, as an external, shared platform, is directly subject to these rules. Its deliverability is contingent upon the cumulative reputation of its shared IPs and the individual sending practices of its users. If authentication is lacking, unsubscribe options are unclear, complaint rates are high, or overall sender reputation is poor, emails sent via SendGrid are likely to be flagged as spam. Conversely, emails originating from O365 benefit from Microsoft's internal trust and established reputation management, as they are often internal communications or from highly reputable senders who naturally meet Microsoft's filtering criteria, thus reducing their likelihood of being routed to spam.

The contrasting deliverability of emails from SendGrid versus Office 365 to Outlook.com hinges on Microsoft's inherent trust in its own ecosystem and the meticulous reputation-building required from external senders. While Office 365 benefits from native DMARC alignment and Microsoft's robust Exchange Online Protection (EOP) automatically trusting internal mail, SendGrid must actively configure comprehensive authentication-SPF, DKIM, and especially DMARC alignment-for its users' domains. EOP assigns a sender reputation score, impacting whether third-party emails reach the inbox or spam. Crucially, SendGrid users must ensure prompt processing of email feedback loops (FBLs) to swiftly remove complainers, as failure to do so rapidly degrades sender reputation and leads to more emails being flagged as suspicious by Outlook.com.