Hard bounce vs soft bounce
A hard bounce is a permanent email delivery failure — the address is invalid, the domain doesn't exist, or the mailbox was deleted — and should be suppressed immediately. A soft bounce is a temporary failure — a full mailbox, a message too large, or a server briefly down — which may succeed on a later retry.
How it works
The receiving server returns a status code: 5xx codes indicate permanent (hard) failures, 4xx codes indicate temporary (soft) ones. Senders retry soft bounces for a while, but a hard bounce means the address will never accept mail and must not be sent to again.
Why it matters
Repeatedly sending to hard-bouncing addresses tells mailbox providers you don't maintain your list, which erodes sender reputation across all your mail. Prompt hard-bounce suppression and pre-send verification are the defenses; ignoring them silently burns your domain.
What should happen to a contact after a hard bounce?
The address goes on a suppression list that every campaign, sequence and sending mailbox checks before a send. Keep the record there instead of deleting it. A deleted contact can come straight back in the next CSV import or enrichment sync, and the second send to a dead address does more harm than the first because it shows the list is not maintained.
The person behind the address may still be reachable. If the contact record holds a phone number or a LinkedIn profile, move them to those channels rather than dropping them, and look for a newer work address if they have changed jobs. A multichannel sequence makes that switch without starting a new campaign.
Why do addresses that passed verification last month bounce today?
Verification is a snapshot. Between the check and the send, people leave companies and their mailboxes are closed, businesses are acquired and move to a new domain, and IT teams tighten which addresses accept outside mail. On a list verified long ago, some hard bounces are expected even if the check was accurate at the time.
Timing of the bounce also varies. Some servers reject during the SMTP conversation, so the failure is logged at once. Others accept the message and send a non-delivery report minutes or hours later, which is common on catch-all domains. A sending system has to catch those late bounce messages too, or the address stays active and gets mailed again.
Which permanent rejections should not remove the contact?
A 5xx code means the receiver will not accept this message as sent, but not every one of them says the address is dead. A 5.3.4 code means the message was too big for the system. A rejection citing content, policy or your sender's reputation is about the message or the sender. Suppressing the contact in those cases hides the real problem and throws away a valid prospect.
Base the suppression rule on the reason, not the first digit: mailbox-does-not-exist and domain-does-not-exist responses go on the list, and everything else gets investigated. The guide to what an email bounce-back means lists the common response texts. Store the full text with the contact record so anyone reviewing it later can see why it was or was not suppressed.
How do you keep hard bounces low before the next campaign?
Verify at import, re-verify any segment that has not been mailed in months, and treat a new list source as a test until its bounce numbers are known. Autocloz suppresses hard bounces automatically and runs email verification before sending, with 1,000 checks a day on the Free plan. Ongoing list hygiene covers the rest of the record.
It also helps to watch hard bounces per list source in the first batch of any new campaign. If the first few hundred sends from a source bounce at several times the rate of your other lists, pause it and verify the rest before the remaining thousands go out, rather than discovering the damage after the whole list has been mailed.
How Autocloz handles it
Autocloz verifies addresses before sending, auto-suppresses hard bounces immediately, and retries soft bounces sensibly — surfacing bounce trends per mailbox so one bad list segment can't quietly damage your reputation.
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FAQ
Should I retry a hard bounce?
No. A hard bounce is a permanent failure — the address is invalid or gone. Suppress it immediately and never resend. Retrying hard bounces signals poor list hygiene and damages sender reputation.
How many soft bounces become a problem?
Occasional soft bounces are normal, but an address that soft-bounces repeatedly over several sends should be treated like a hard bounce and suppressed. Persistent soft bounces often indicate an abandoned or problematic mailbox.
Related terms
Email bounce rate is the percentage of sent emails that are returned undelivered. Hard bounces are permanent (invalid or non-existent address); soft bounces are temporary (full mailbox, server down). A high bounce rate signals a poor list and damages sender reputation.
A spam trap is an email address operated by mailbox providers and blocklist operators specifically to catch senders with poor list hygiene. There's no real person behind it, so any mail it receives indicates you're emailing addresses you didn't earn permission to contact.
Email verification is the process of checking whether an email address is valid and safe to send to — confirming the syntax, domain and mailbox exist and aren't a known trap or disposable address — before you add it to a campaign. It is the single most effective way to control bounce rate.
List hygiene is the ongoing practice of keeping an email list clean and accurate — verifying addresses, removing invalid and hard-bounced contacts, suppressing unsubscribes and long-term non-engagers, and avoiding purchased or scraped data. Good hygiene protects deliverability and keeps sender reputation healthy.