In a load balanced scenario which service would require persistence to function correctly?

SMTP is a stateless, transaction-based protocol where each message delivery is a self-contained exchange that does not require affinity to a specific server.

DNS uses short-lived, stateless UDP queries where each resolution request is independent and persistence provides no functional benefit.

Web-based email applications maintain server-side session state after login, meaning subsequent requests from the same user must be directed to the same back-end server to avoid authentication failures or data loss. Without persistence (such as cookie-based or source-IP affinity), a load balancer could route requests to a server with no knowledge of the established session, breaking the application. Other choices are stateless protocols where each transaction is independent.

Standard HTTP is a stateless protocol by design, and basic HTTP content delivery does not inherently require session persistence to function correctly.