Deliverability

Email bounce codes explained: hard bounces, soft bounces and what to do

Inboxlogy6 min readUpdated 2026-10-26

In short: A bounce is an email that could not be delivered. A hard bounce is permanent (the address does not exist) and you must stop mailing it. A soft bounce is temporary (mailbox full, server busy) and may succeed on retry. The SMTP code in the bounce tells you which. Decode any bounce with the free Bounce Analyzer.

On this page

  1. Hard bounce vs soft bounce
  2. What SMTP codes mean
  3. Why bounces matter for cold email
  4. How to decode a bounce

Hard bounce vs soft bounce

The core distinction:

  • Hard bounce: a permanent failure. The address is invalid, the domain does not exist, or the recipient is blocked. These must be removed from your list immediately; mailing them again damages your reputation.
  • Soft bounce: a temporary failure. The mailbox is full, the server is down, or the message is too large. These may deliver on retry, but repeated soft bounces should also be pruned.
Key point: High hard-bounce rates are one of the fastest ways to wreck sender reputation and land on a blacklist. Mailbox providers read lots of hard bounces as a sign you are mailing a purchased or stale list.

What SMTP codes mean

Bounces carry a three-digit SMTP code. The first digit tells you the category:

  • 5xx: permanent failure (hard bounce). For example 550 (mailbox unavailable), 550 5.1.1 (user unknown).
  • 4xx: temporary failure (soft bounce). For example 421 (service unavailable), 452 (mailbox full).

The enhanced status code (like 5.1.1) narrows it further to the exact reason.

Why bounces matter for cold email

Cold email lives and dies by list quality, and bounces are the clearest signal of a bad list. A high bounce rate tells mailbox providers you did not verify your recipients, which is classic spammer behaviour. Keeping bounces low, by verifying addresses before sending and pruning hard bounces immediately, protects the reputation your whole campaign depends on.

How to decode a bounce

Bounce messages are often cryptic. Paste the bounce text or SMTP code into the Bounce Analyzer and it tells you whether it is hard or soft, what the code means, and what to do. If bounces are frequent, check your blacklist status too, since the two problems often travel together.

Decode any bounce message free

Paste a bounce or SMTP code and see whether it's hard or soft, what it means and how to fix it.

Open the Bounce Analyzer →All free tools

Frequently asked questions

What is the difference between a hard and soft bounce?
A hard bounce is a permanent failure, such as an address that does not exist; remove it from your list at once. A soft bounce is temporary, such as a full mailbox, and may deliver on retry.
What does a 550 bounce code mean?
550 is a permanent failure, usually meaning the mailbox is unavailable or the user is unknown (often shown as 550 5.1.1). Treat it as a hard bounce and stop mailing that address.
Do bounces hurt my sender reputation?
Yes, especially hard bounces. A high hard-bounce rate signals a stale or purchased list, which mailbox providers penalise. Verify addresses before sending and prune hard bounces immediately.
Is the Bounce Analyzer free?
Yes. Paste a bounce message or SMTP code and it explains the cause and fix in your browser, at no cost and without an account.

Keep reading

Blacklist CheckerCheck an IP/domain against DNSBLsSPF CheckerValidate SPF + the 10-lookup limitDNS Record CheckerLook up A, MX, TXT, NS in one clickWarm-Up CalculatorPlan a domain warm-up ramp