Building a Functional Email Server with Dominick LoBraico

Faced with the security risks and operational rigidity of a legacy mail server, Jane Street engineers replaced the system with MailCore, a homegrown server implemented in OCaml where configuration is expressed as composable code. To ensure safety during the one-year migration, the team deployed a parallel shadow instance that continuously diffed outputs against the legacy system, effectively reverse-engineering implicit knowledge while eliminating memory-safety bugs common in C-based alternatives. This architectural shift transformed email management into a software culture practice, allowing generalist engineers to safely modify routing and filtering logic using standard OCaml tools rather than relying on specialized knowledge of opaque custom domains.