421 and 451 email errors are temporary failures (4xx codes): the receiving server said "not now, try again later". Your mail server keeps the message in its queue and retries automatically, usually for several days, before giving up with a bounce. Many deferrals clear on their own, but repeated ones point to a fixable problem.
Before you start
- Find the full deferral text in cPanel → Email → Track Delivery or in the "delayed delivery" warning you received.
- Note the remote server and the time of each attempt.
Common 421 and 451 messages
| Message | Meaning |
|---|---|
| 451 Greylisted, please try again later | The receiver rejects first attempts from unknown senders on purpose. The retry succeeds. |
| 421 Too many connections / rate limited | You sent too much too fast to that provider. |
| 421 4.7.0 … temporarily deferred due to unexpected volume or user complaints | Reputation-based throttling (common with large providers). |
| 451 Temporary local problem / DNS lookup failed | The receiver could not check your domain's DNS (SPF, MX, reverse DNS) at that moment. |
| 451 Mailbox temporarily unavailable | The recipient's mailbox is locked or over quota. |
How to fix temporary email errors
- Wait for retries if the message is greylisting or a one-off. Most arrive within 15–60 minutes.
- Slow down bulk sending. Spread mail out and stay within your hosting's hourly limit; see email sending limits.
- Check your DNS health. In cPanel → Email → Email Deliverability, make sure SPF and DKIM are valid. Broken or very long SPF records cause lookup failures.
- Improve reputation if a big provider throttles you: remove inactive recipients, make unsubscribing easy and avoid sudden volume spikes.
- Check for spam from your account. A compromised mailbox or script sending spam triggers throttling for everyone on the IP. Change passwords and review the mail queue.
- Contact support with the deferral text if messages stay deferred for hours to one provider.
Note: A 4xx error is not a bounce. Do not resend the message manually while it is still queued, or the recipient may get it twice.
FAQ
How long will the server keep retrying?
Typically several days, with increasing gaps, before returning a final bounce.
Is greylisting a problem I need to fix?
No, it is the receiver's anti-spam method. Legitimate mail is delivered on retry.
What is the difference between 4xx and 5xx?
4xx is temporary and retried; 5xx is permanent, like 550 errors.