How to Use Sub-Domains to Improve Email Deliverability in 2025

In 2025, email deliverability has become increasingly challenging due to stricter rules from providers like Gmail, Yahoo, and Outlook, which now evaluate sender reputation at the sub-domain level, rather than the entire domain. To mitigate risks, companies are advised to segment their email traffic across dedicated sub-domains, such as using notify.example.com for transactional emails and newsletter.example.com for marketing messages. This strategy helps protect critical user flows from being affected by issues with marketing campaigns, facilitates compliance with authentication protocols like SPF, DKIM, and DMARC, and allows for more flexible and secure management of email streams. Teams adopting this approach have reported a significant reduction in emails being flagged as spam, with a 24% lower spam folder rate, enhanced security, and the ability to experiment with marketing strategies without jeopardizing transactional email reliability. As more organizations recognize the benefits of sub-domain segmentation, it is becoming a standard practice for maintaining email deliverability and protecting brand reputation.