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.
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.
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.