A bounce is a delivery failure notification returned by a mail server. Hard bounces are permanent. Soft bounces are temporary. Both damage your sender reputation if left unfixed.
An email bounce is an automated failure message returned by a receiving mail server when it cannot deliver a message to the intended recipient. The SMTP server returns a 5xx (permanent) or 4xx (temporary) status code with a reason. The sending server records this as a bounce and (usually) removes or flags the address.
Permanent failure. The address does not exist, the domain has no mail server, or the server explicitly rejects the sender.
550 5.1.1 — User doesn\'t exist550 5.7.1 — Rejected (policy)521 5.2.1 — Mailbox disabledTemporary failure. The address may be valid but the server is full, unavailable, or throttling your IP.
452 4.2.2 — Mailbox full421 4.7.0 — Try again later451 4.3.5 — Server error (temp)Gmail tracks bounce rate at the sending IP and domain level. Above 2% bounce rate over a rolling window, your domain enters a throttle queue. Messages take hours to deliver — or land in spam.
Microsoft's Smart Network Data Services (SNDS) scores your IP on complaint rate and bounces. A damaged SNDS score means junk folder placement across all Outlook, Hotmail, and Live addresses globally.
Mailchimp, Klaviyo, and SendGrid monitor bounce rates. Breaching 2% triggers account warnings. Repeated violations lead to account suspension — even if the bounce was caused by buying a list one time.
Major ISPs exchange data through feedback loop programmes. A poor reputation with one can cascade to others. Removing hard bounces immediately limits cross-ISP contamination.
Run every list through email verification before any campaign. BounceZero catches invalid, disposable, role-based, and catch-all addresses. Removes the hard bounces before they happen.
Double opt-in confirms the email address belongs to a real person who controls it. Eliminates typos and fake entries at the source.
Immediately add hard-bounced addresses to a global suppression list. Never send to them again — even if a contact re-imports them months later.
Corporate emails expire when employees leave. Consumer emails get abandoned. Lists degrade at 20–25% per year. Regular verification before each campaign keeps bounce rate below 0.5%.
Sending high volume from a cold IP triggers temporary 4xx soft bounces from throttling. Ramp volume 20–30% per day over 4–6 weeks before full-scale sending.
Google Postmaster shows your domain reputation and IP reputation in real time. Set up daily alerts. Catch reputation damage before it becomes a deliverability crisis.
A hard bounce is a permanent delivery failure — the address does not exist, the domain has no mail server, or the server explicitly rejects your messages. A soft bounce is a temporary failure — full mailbox, server unavailable, or rate limiting. Hard bounces should be removed immediately. Soft bounces should be retried 2–3 times then suppressed.
The industry benchmark is below 0.5% for well-maintained lists. Gmail and Outlook begin throttling senders above 2%. Most ESPs (Mailchimp, Klaviyo, SendGrid) will warn or suspend accounts that consistently exceed 2%. Lists sent without prior verification routinely hit 5–15% bounce rates.
Check the bounce notification email returned to your sending address or ESP bounce log. It will contain an SMTP status code (5xx for hard, 4xx for soft) and a reason phrase. Common hard bounce codes: 550 5.1.1 (user not found), 550 5.7.1 (policy rejection). Common soft bounce: 452 4.2.2 (mailbox full), 421 4.7.0 (try later).
Verification eliminates hard bounces caused by invalid addresses — typically 5–15% of unverified lists. It cannot eliminate soft bounces from server-side throttling or temporary failures. Catch-all domains (which accept all addresses regardless of validity) are flagged as "Risky" rather than definitively verified.
Verify before every major campaign and any list that has not been sent to in more than 3 months. Corporate email lists decay at 2% per month (employees change jobs). Consumer lists decay more slowly but still degrade 15–25% per year. Verification before each send keeps bounce rate consistently below 0.5%.
Deep-dive guides on how email verification and inbox placement work
How the 5-stage pipeline checks a mailbox without sending email
The RCPT TO handshake explained — codes, catch-alls, limits
Why domains accept everything and how ML classifies deliverability
Temp addresses, how to detect them, and why they matter
Get from 5-15% to under 0.5% in 6 steps
Authentication, list hygiene, IP warmup, ISP monitoring
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