What is a deferred message (4xx error)?

A deferred message is one that received a 4xx SMTP response code, indicating a temporary failure. The recipient server is saying "I can't accept this right now, but try again later." The message moves to your deferred queue awaiting retry.

Common 4xx errors:

421: Service not available, closing connection (server overloaded or maintenance)

450: Mailbox unavailable (temporary condition)

451: Local error in processing (server-side issue)

452: Insufficient storage (mailbox or server full)

Unlike permanent rejections (5xx), deferred messages have hope. The problem may resolve: server restarts, storage clears, rate limits reset. Your MTA retries according to your retry policy.

High deferral rates to a specific domain often indicate rate limiting. The provider is throttling your sending speed. High deferrals across all domains suggest an issue with your sending IP or content.

Monitor your deferred queue size. A small number is normal; every sender experiences temporary issues. A growing backlog signals problems needing investigation.

Deferral is the port authority saying "the dock is full, anchor offshore and wait for your slot."