What is a bounce?
A bounce happens when an email can’t be delivered to the recipient. The receiving mail server sends back a notification explaining why the delivery failed. Bounces are a normal part of sending email, but high bounce rates damage your sender reputation.Types of bounces
Hard bounce
A hard bounce is a permanent delivery failure. The email will never be deliverable to this address. Common causes:
What to do: Remove hard-bounced addresses immediately. Never send to them again. Continuing to send to hard-bounced addresses signals to email providers that you don’t maintain your list.
Soft bounce
A soft bounce is a temporary delivery failure. The email might be deliverable if you try again later. Common causes:
What to do: SendKit automatically retries soft bounces. If an address soft bounces 3 times, SendKit automatically suppresses it — treating it as a hard bounce. On successful delivery, the soft bounce counter resets.
Bounce codes
Bounce notifications include SMTP status codes that explain the failure. The codes follow a pattern:
The first digit indicates the category:
- 4.x.x — temporary failure (soft bounce)
- 5.x.x — permanent failure (hard bounce, with some exceptions)
Bounce handling in SendKit
SendKit handles bounces automatically:Hard bounces
- The email is marked as bounced in your dashboard
- The
email.bouncedwebhook is triggered - The address is added to the suppression list — future sends to this address are automatically blocked
Soft bounces
- SendKit retries delivery automatically
- If delivery succeeds on retry, the email is marked as delivered and the soft bounce counter for that address resets
- If all retries fail, the email is marked as bounced
- After 3 soft bounces to the same address, it’s automatically added to the suppression list
The suppression list
The suppression list prevents you from sending to addresses that have previously hard bounced. This is critical for maintaining your reputation. If you try to send to a suppressed address, SendKit blocks the send and returns an error.Bounce rate thresholds
SendKit evaluates reputation with a minimum of 200 emails in the last 24 hours. If your bounce rate exceeds 10%, sending is automatically paused and your team admins are notified.
Preventing bounces
At collection
- Validate email addresses at signup — use the SendKit email validation API to catch typos and invalid addresses before they enter your list
- Use double opt-in — send a confirmation email and only add contacts who confirm. This eliminates typos and fake addresses
- Don’t accept catch-all addresses blindly — some domains accept all emails regardless of the mailbox. These can still bounce later
Ongoing
- Clean your list regularly — remove addresses that haven’t engaged in 90+ days
- Monitor bounce metrics — check your dashboard after every campaign or batch send
- Honor suppression lists — never circumvent the suppression list
FAQ
What's a good bounce rate?
What's a good bounce rate?
Under 2%. If you’re consistently above 2%, you need to improve your list hygiene. Above 5% is critical and can result in email providers throttling or blocking your emails.
Should I retry hard bounces?
Should I retry hard bounces?
No. Hard bounces are permanent. Retrying wastes resources and hurts your reputation. Remove hard-bounced addresses from your list immediately.
Why did an address that used to work start bouncing?
Why did an address that used to work start bouncing?
Email addresses become invalid over time. People leave companies, abandon personal accounts, or reach storage limits. This is why regular list cleaning is important — even a “clean” list degrades over time.
Can I get unbounced addresses back?
Can I get unbounced addresses back?
If a hard-bounced address is now valid again (e.g., the mailbox was recreated), you can remove it from the suppression list in your SendKit dashboard. But verify it first — don’t assume.