What’s the difference between temporary and permanent blocks?
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You're staring at your delivery logs and seeing a mix of 4xx and 5xx errors. Both are rejections, but they mean very different things and need very different responses.
Think of it this way. A 4xx code is the receiving server saying "not right now, but try again later." A 5xx code is the receiving server saying "don't bother, this isn't happening."
Temporary blocks (4xx codes) are short-term. The receiving mail server is still willing to accept your message. It's just not ready at this moment. Common reasons include rate limiting (you're sending too fast), greylisting (an intentional delay for first-time senders), or a server that's temporarily overloaded. Your sending setup should hold the message and try again after a waiting period, usually starting at a few minutes and stretching longer with each attempt. That pattern is called retry backoff.
Common 4xx codes you'll see:
- 421. Service temporarily unavailable
- 451. Temporary local problem, try later
- 452. Insufficient storage on the receiving side
Permanent blocks (5xx codes) are final. The receiving server has made a decision and it's not going to change just because you retry. The address might not exist, your content may have triggered a spam filter, or your IP or domain is on a blocklist. Retrying a 5xx doesn't just waste effort. It can actively hurt your sender reputation because it looks like you're ignoring the rejection.
Common 5xx codes you'll see:
- 550. Mailbox not found or user unknown
- 551. User not local
- 553. Mailbox name invalid
- 554. Transaction failed (often spam-related)
The practical split is simple. A 4xx means queue it and wait. A 5xx means suppress the address and stop. If you keep hammering a 5xx address, mailbox providers notice. And not in a good way.
Still if you're seeing a flood of 5xx errors on addresses you thought were valid, it's worth running your list through a validation pass. You can clean your list with Review My Emails, or if you're troubleshooting a live delivery problem right now, our SOS hotline is free.
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