In the cloud era, hosting-based email services have become a common business model. Various entities can participate in the email delivery process. However, the intermediate paths of email delivery have received little attention. In particular, the vulnerabilities and centralization of email intermediate paths have already posed real-world security threats. This paper conducts the first systematic analysis of intermediate paths of email delivery, aiming to understand dependence patterns and characterize the centralization. In collaboration with a large email service provider, we collected Received headers from email reception logs spanning nine months and reconstructed the complete intermediate paths of 105M clean emails. Our results reveal that Microsoft is the dominant provider of intermediate paths, participating in 66.4% of emails. We find that 86.9M (82.7%) emails rely on third-party providers in intermediate paths, and 9.1M (8.7%) paths involve multiple providers. Email signature providers frequently appear in cross-vendor intermediate paths. In addition, we reveal significant differences in the regional dependencies and centralization of email intermediate paths across countries and continents. The centralization observed in email intermediate paths also differs from incoming and outgoing servers. We hope our work prompts more attention to email intermediate paths to enhance the security of the email ecosystem.

Abnormal email bounces seriously disrupt user lives and company transactions. Proliferating security protocols and protection strategies have made email delivery increasingly complex. A natural question is how and why email delivery fails in the wild. Filling this knowledge gap requires a representative global email delivery dataset, which is rarely disclosed by email service providers (ESPs).