Mail queue

What it means

RFC 5321 requires a sender to queue mail that cannot be transmitted at once and to retry it periodically; a queue entry holds the message and its envelope. Messages wait there after a 4xx reply, a failed connection or a DNS error, until a retry succeeds, a 5xx reply ends the attempt, or the queue lifetime runs out and the message is bounced.

A growing queue is often the first sign of trouble. When one destination starts deferring, mail for it piles up while everything else flows, and the sender sees delays before any bounce. Postfix ships qshape to show which destinations a queue is backing up on, and Yahoo advises operators who see deferrals to examine their outbound queues in case spammers are abusing the server.