When Delivery Meets Error: Exploring Email Delivery Retry Strategies and Defects
Abstract
Reliable delivery is a basic expectation of email services. The increasingly complex ecosystem has led to more frequent email bounces. When encountering various errors, properly handling failures and retrying are crucial to ensuring email deliverability. Currently, the community lacks a comprehensive understanding of email retry strategies, and bridging this gap can enhance the reliability of email services. In this paper, we measure the email delivery retry strategies of 11 email service providers and 8 open-source email software. We deploy a platform (RetryWatch) that simulates various types of bounce errors returned by email receiving servers, covering the DNS, TCP, and SMTP communication stages. We find that many providers and software exhibit defects in their email retry implementations. For example, we find that six email systems exhibit poor load balancing across MX record entries in dual-stack environments. In addition, when encountering blocklist- and greylist-related SMTP error messages, most email providers fail to handle retries appropriately. We release RetryWatch to support further testing by the email community. We hope this work helps improve email reliability and inspires efforts to refine email retry strategies.
