Retry schedule

What it means

RFC 5321 sets the frame. After a failed attempt the sender must wait before trying the same destination again; the retry interval should generally be at least 30 minutes, and the overall give-up time generally at least 4 to 5 days. It observes that two attempts in the first hour, then one every two or three hours, suits typical transient failures. MTAs make this configurable; Postfix, for example, sets the range of waits with minimal_backoff_time and maximal_backoff_time.

Retry timing interacts with receivers' defences. Greylisting refuses a new sender until a minimum delay has passed, so a sender that never retries never gets through, and providers that throttle expect senders to slow down rather than reconnect at once. Gmail's guidance after a 4.7.28 rate limit is to stop sending for at least 10 minutes, then resume on a single connection.