What are common 4xx codes and their meanings? (e.g., 421, 451)

You're sending emails and some are bouncing back with 4xx codes. Good news: these aren't rejections. They're the receiving server telling you "not right now, try again later." The trick is knowing what each one actually means so you can handle retries the right way.

Here's a rundown of the most common ones you'll see in the wild.

421 Service not available
The receiving server is too busy, rate-limiting you, or temporarily shutting down. This one often shows up when you're sending volume too fast to a mailbox provider like Gmail or Outlook. Back off, slow down your sending rate, and retry. If you're seeing 421s in bulk, it's usually a signal to throttle your send speed rather than just queue and blast again.

450 Mailbox unavailable
The mailbox is temporarily busy, locked, or blocked. You'll also see this from greylisting, where a server rejects the first attempt from an unknown sender on purpose and expects a legitimate mail server to retry. Most ESPs retry automatically and the email goes through on the second attempt.

451 Local error in processing
The receiving server hit an internal problem while trying to accept your message. It's not your fault. That said, some servers also use 451 to flag a content or reputation issue without saying so outright. If you're getting persistent 451s from the same domain, it's worth checking whether your sender reputation is involved.

452 Insufficient storage
The server or the recipient's mailbox is temporarily full. This usually clears on its own as the user cleans up their inbox. Standard retry logic handles it fine.

454 TLS not available
A temporary TLS or encryption issue on the receiving end. Retry typically resolves it. If it persists, the receiving server may have a misconfigured certificate.

For all 4xx codes, the right response is the same: queue the message, use exponential backoff (wait longer between each retry attempt), and convert to a permanent failure only after your retry window is exhausted. Most ESPs do this automatically. Where it gets nuanced is with 421s specifically. If they're coming in volume, just retrying isn't enough. You need to slow down the whole sending stream to that domain, not just retry individual messages.

If 4xx codes are piling up and you're not sure whether it's a reputation issue or a server-side problem, our SOS hotline is free and we'll help you read the signals.