A checkout typo and a full Gmail inbox both show up as “bounced” in the campaign report. One of those addresses should never be mailed again. The other is a customer who will receive the shipping email tomorrow if you leave them alone.
The report collapses both into one rate. The SMTP conversation does not.
Last verified: 26 September 2026, against Klaviyo’s bounce and suppression documentation and RFC 5321.
Hard bounce vs soft bounce: what each one is
Internet mail defines the classes. Your ESP defines the labels.
| Hard bounce | Soft bounce | |
|---|---|---|
| SMTP class | Permanent negative completion, a 5xx code (RFC 5321) | Transient negative completion, a 4xx code |
| Typical cause | Mailbox does not exist, domain does not accept mail, policy rejection of that recipient | Mailbox full, server busy, greylisting, temporary rate limit |
| What to do with the address | Suppress it | Retry. Suppress only if it keeps failing |
| What it is not | Proof the customer marked you as spam | Proof the address is fake |
Klaviyo’s own definitions match that split. A hard bounce is a permanent reason, including a misspelled address, and Klaviyo suppresses it. A soft bounce is a temporary reason, such as a full inbox or a server that is down for a moment. That wording is Klaviyo’s suppression policy, not a Gmail or Yahoo requirement.
How to read the numeric codes, including the cases where a 5xx is a policy block rather than a bad address, is in SMTP 4xx vs 5xx. A 4xx that your ESP keeps retrying is a deferral.
What Klaviyo does with each one
Klaviyo suppresses a hard bounce immediately. It suppresses a soft bounce only after more than seven consecutive soft bounces, counting soft bounces in the past two years. Those deliverability suppressions stay in place if the person later fills out a form. Klaviyo’s suppression article says a resubscribe lifts other suppressions, not a hard-bounce suppression.
There is a second Klaviyo-only signal. An address that has hard bounced at least once anywhere on Klaviyo’s infrastructure can be marked suspicious and skipped, even if it has never bounced in your account. That is Klaviyo protecting the shared infrastructure. It is not a mailbox provider telling you the customer complained.
If you export the list and upload it to another ESP, those suppressions do not travel with the file unless you export them. The ESP migration guide is the cutover. This page only decides which addresses are dead.
When a soft bounce should start to worry you
One soft bounce is normal. A wave of soft bounces at one provider, on the same day, usually means that provider is deferring or throttling you. Cleaning the list does not fix a throttle. Slowing down and reading the 4xx text does.
On Klaviyo, a soft-bouncing address gets more than seven consecutive failures before it is suppressed, while a hard bounce is suppressed on the first. That is Klaviyo’s policy, not a mailbox provider rule, and other ESPs count differently. Repeated soft bounces on the same addresses are a list problem. Those mailboxes are full, abandoned, or rejecting you specifically. Let the seven-bounce suppression happen, or suppress the chronic ones sooner with a segment. Do not “re-engage” them to prove the address works. A win-back to chronic bouncers manufactures the next hard bounce.
What a bounce does not tell you
Gmail’s published spam-rate guidance is about people who received the mail and marked it spam. A bounce never entered that calculation, because the message was not accepted into the mailbox. Yahoo’s complaint rate is also calculated on mail that reached the inbox. If the business question is “are we in spam?”, use bounce rate vs complaint rate and the provider’s own complaint view, not the bounce tile.
Amazon SES counts bounce rate from hard bounces only, recommends staying under 2%, and publishes different thresholds than Klaviyo’s in-product bands. Those SES figures live on the SES deliverability page. Do not paste a Klaviyo “healthy bounce” band onto an SES account, or the reverse. To see your own number, run your send and hard-bounce counts through the bounce rate calculator. If it is already climbing, the high bounce rate fix page is the runbook.
InboxEagle
Turn your bounce counts into a rate
Enter sends, hard bounces, and soft bounces to see your rate against the 2% line SES recommends. Then check which provider the failures share before you clean the whole list.
Sources
- SMTP reply codes, RFC 5321. 4xx is transient, 5xx is permanent.
- Understanding bounced emails, Klaviyo. Hard bounce suppression and the seven consecutive soft bounces.
- Understanding suppressed profiles, Klaviyo. Deliverability suppressions are not removed by a resubscribe.