Bounce messages are the most useful diagnostic information in cold email and the most commonly ignored. Teams see “delivery failed”, mark the address bad and move on, when the error text usually names the exact problem.
Here are the codes that matter, what each is really saying, and what to do about it.
How to read any SMTP code
Every reply has a three-digit code and usually an enhanced status code, like 550 5.7.1. The first digit is the important part.
| Starts with | Meaning | What to do |
|---|---|---|
| 2xx | Accepted | Nothing. Accepted is not the same as inboxed, though. |
| 4xx | Temporary failure, soft bounce | Retry later. Investigate if it repeats. |
| 5xx | Permanent failure, hard bounce | Remove the address. Fix the cause if it is your side. |
The enhanced code after it narrows things down. 5.1.x is about the address, 5.7.x is about policy and authentication. In cold email, 5.7.x is where your real problems live, because those are the ones caused by your configuration rather than a bad address.
550 5.7.1, the one everyone hits
The message was rejected for policy reasons. The receiving server understood you perfectly and chose not to deliver. There are four common causes and they need different fixes.
Cause 1: authentication failed
SPF, DKIM or DMARC did not pass. Send a test to a Gmail address, open the original and read the authentication results. If any line says FAIL, that is your answer. Fix the DNS record, wait for propagation, retest.
Cause 2: your IP or domain is blocklisted
Check the sending IP and domain against the major blocklists. If you are listed, the rejection text usually names the list and links to the delisting process. Delist, then fix whatever caused the listing, because relisting is faster the second time.
Cause 3: the recipient’s server has a rule against you
Corporate filters block by sender domain, by keyword, or by reputation score. Nothing you change in DNS will help. This is a signal that your domain reputation with that organisation is poor, or that your content triggered a rule.
Cause 4: relay denied
You are trying to send through a server that will not relay for you. This is a configuration error in your sending setup rather than a reputation problem, and it usually appears immediately after changing providers.
The other codes worth knowing
| Code | What it means | Fix |
|---|---|---|
| 550 5.1.1 | Address does not exist | Remove it. If you see many, your list needs verification before the next send. |
| 550 5.7.26 | Failed Google bulk sender authentication | Publish or correct SPF, DKIM and DMARC, and check From-domain alignment. |
| 550 5.7.606 | Microsoft: IP blocked by tenant or reputation | Use Microsoft’s delisting portal, then investigate why the IP was flagged. |
| 421 4.7.0 | Throttled, sending too fast | Reduce per-mailbox rate, spread across more mailboxes, ramp more slowly. |
| 452 4.2.2 | Recipient mailbox full | Retry for a few days, then treat as inactive. |
| 554 5.7.1 | Rejected as spam at connection | Serious. Check blocklists and content, and pause the campaign on that domain. |
What your bounce pattern is telling you
A single bounce is noise. The pattern is the diagnosis, and it is worth spending two minutes on before you change anything.
- Bounces concentrated on one mailbox or domain, the problem is that mailbox or domain, not your list. Check its DNS and blocklist status.
- Bounces spread evenly across all mailboxes, the problem is your list. Verify it before sending again.
- Bounces from one provider only, a reputation problem with that provider specifically. Common with Microsoft 365 tenants.
- Bounce rate rising over days, usually a reputation slide rather than a list issue. Pause, reduce volume, and investigate.
- Mostly 421s, you are sending too fast for your current reputation. This is the easiest one to fix and the most commonly ignored.
This is also the argument for infrastructure that reports per mailbox and per domain rather than per campaign. If all you can see is a campaign-level bounce rate, you cannot tell case one from case two, and those need opposite responses.
Prevention, in order of impact
- Verify the list before every send. Most hard bounces are avoidable. Email verification catches them before they cost you reputation.
- Get authentication right once, automatically. Manual SPF and DKIM across dozens of domains is where
5.7.xerrors come from. Automate the publishing and the updates. - Ramp volume gradually. 421 throttling is a self-inflicted wound.
- Send to people who might plausibly buy. Bad-fit lists produce complaints, complaints produce reputation damage, and reputation damage produces
5.7.1. - Watch bounce rate as a leading indicator. Under 2% is healthy. Above 5%, stop and fix it before you burn a domain.
Next: how to reduce your bounce rate, or Google’s bulk sender requirements in full.
Frequently asked questions
What does 550 5.7.1 mean?
It is a permanent rejection on policy grounds. The receiving server accepted the connection, looked at your message, and decided not to deliver it. The most common causes are failed authentication, a blocklisted IP or domain, or a recipient-side rule blocking your sender. It is not a full mailbox and it will not resolve on retry.
What is the difference between a soft bounce and a hard bounce?
A hard bounce is permanent, usually a 5xx code, and means the address does not exist or delivery was refused outright. A soft bounce is temporary, usually 4xx, and means try again later, typically because of throttling or a full mailbox. Hard bounces should be removed from your list immediately; soft bounces can be retried a few times before you give up on them.
What does 5.7.26 mean?
The message failed authentication under Google’s bulk sender requirements. It means SPF, DKIM or DMARC did not pass, or the From domain was not aligned with the authenticated domain. Fix the DNS records rather than the copy.
Why am I getting 421 errors?
You are being throttled. The receiving server is telling you it is accepting mail but not at the rate you are sending it. Slow down, spread sends across more mailboxes, and check whether you increased volume recently. Persistent 421s from one provider usually mean your reputation with them is deteriorating.
What bounce rate is acceptable for cold email?
Under 2% is healthy, 3% is a warning, and above 5% you should pause and clean the list before you damage the domain. High bounce rates are read by inbox providers as evidence you are sending to a purchased or scraped list.