Hard bounces (the recipient doesn't exist) are a list-quality issue: verify your list with an email verification service before sending. Soft bounces (full mailbox, temporary problem) are retried automatically and usually resolve on their own. Rejections with 5xx codes from the recipient's mail server come with an error message that usually says why: spam filtering, authentication, or rate limits. Your sending tool shows these, because the bounce notice comes back to the sending mailbox.
Bounces: types and fixes
Hard bounce (5xx): the recipient doesn't exist, never existed, or is permanently disabled. Fix: remove the address from your list. Don't retry.
Soft bounce (4xx): a temporary issue such as a full mailbox, a server problem, or greylisting. Fix: nothing, usually. Temporary failures are retried automatically, and you only get a bounce if delivery keeps failing.
Content rejection (550 with "spam" in the message): the provider's spam filter blocked it. Fix: improve the content, warm longer, and check the domain's reputation.
Authentication rejection: SPF, DKIM, or DMARC failed. Fix: check the domain's status on the Domains page. A red status icon means a DNS problem to fix. On domains connected with manual DNS records, check the records at your DNS provider are still in place.
Rejections at the recipient server
Common SMTP error codes:
- 421: try again later. Rate limiting or a temporary issue. Slow down.
- 450: mailbox temporarily unavailable. Similar to 421.
- 550: user unknown or permanent failure. Remove the address from your list.
- 551, 552, 553: permanent failures for various reasons (no such user, mailbox over quota, invalid address). Remove the address from your list.
- 554: transaction failed, often spam-related. Reduce volume and check content.
Read the full error text in the bounce message. The recipient's server usually says exactly why.
Fixing list quality
- Verify before sending. Every batch. Verification services are cheap and catch a meaningful share of dead addresses.
- Segment your list. Don't keep emailing contacts who never engage.
- Suppress hard bounces immediately. Once an address hard-bounces, add it to your sending tool's suppression list and never send to it again.
When to stop a campaign
Watch for these while a campaign is running:
- Bounce rate rising past 3%: pause and investigate.
- 550 spam errors from multiple providers: likely a blocklist or reputation issue. Pause and check.
- 421 rate limits piling up: you're sending too fast to that provider. Slow down.
What's next
Frequently asked questions
What's an acceptable bounce rate?
Under 2% is healthy. 2-5% suggests list quality is degrading. Above 5% starts causing real deliverability damage.
Do bounces damage my reputation?
Yes. A high bounce rate tells inbox providers you're sending to a poorly maintained list, and they filter you more aggressively. That's why list verification is worth the cost.
Where do I see bounce messages?
A bounce comes back as a delivery-failure email to the mailbox that sent the message, so your sending tool (and the mailbox itself) sees it. The Winnr app doesn't have a separate bounce report. If you use webhooks, see [Webhooks](/help/api-mcp/webhooks.html) for the delivery events available.
What's the difference between a bounce and a rejection?
A rejection happens during delivery, when the recipient's server refuses the message outright (an SMTP 5xx error, usually spam filtering, authentication, or an unknown user). A bounce is the notice you get back afterwards saying the message couldn't be delivered. In practice both reach you as a delivery-failure email with the receiving server's error message.
My messages get 550 "user unknown" errors. What now?
The recipient address doesn't exist. Remove it and verify your list. If it's a small percentage of a campaign, just clean those addresses out. If it's widespread, stop the campaign and clean the whole list.
I'm getting 421 "try again later". What now?
A temporary rate limit from the recipient's server. Slow your sending pace. If it's happening across many recipients at the same provider, halve your daily volume to that provider and monitor.