What is a deferred message (4xx error)?
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Your ESP dashboard shows a spike in 4xx errors after a big send. Should you panic? Not yet. A deferred message is just an email that got a temporary rejection from the receiving server. Think of it as the server saying "not right now, try me again later." The message doesn't bounce. It sits in your deferred queue and your MTA retries it automatically.
The 4xx code tells you the failure is temporary, not permanent. That's the key distinction. A 5xx error means permanent rejection. A 4xx means there's still a chance.
Common 4xx codes you'll see:
- 421. Service not available. The server is overloaded or in maintenance and closed the connection.
- 450. Mailbox unavailable right now (a temporary condition on their side).
- 451. Server-side processing error. Something went wrong on the receiving end.
- 452. Insufficient storage. Their mailbox or server is full.
Most of the time, these resolve on their own. The server restarts, storage clears, rate limits reset. Your MTA follows its retry policy and keeps trying at set intervals until delivery succeeds or the message expires.
What the pattern of deferrals tells you matters a lot. High deferrals to one specific domain (say, everyone on Gmail or Outlook) usually means that provider is rate limiting you. They're throttling how fast they'll accept your mail. That's not necessarily a reputation problem. It might just mean you sent a large volume quickly and hit their inbound limits.
High deferrals spread across every domain is a different story. That pattern points to something wrong on your side: a sending IP with reputation issues, content triggering filters, or authentication problems worth investigating.
But a small number of deferrals in any given send is completely normal. Every sender hits temporary issues. What you want to watch for is a growing deferred queue that isn't draining. That's the signal to dig deeper into what's actually blocking delivery.
If your deferrals keep climbing and you're not sure why, our SOS hotline is free and we'll help you read the signals without a sales pitch.
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