Soft bounce vs hard bounce: what actually differs
A hard bounce is permanent. A soft bounce is temporary. That's the headline. The interesting part is the SMTP codes, the retry behavior, and what each one does to your sender reputation if you ignore it.
A hard bounce is a permanent delivery failure (SMTP 5xx) — the address does not exist or you've been blocked. Suppress immediately. A soft bounce is a temporary failure (SMTP 4xx) — full mailbox, server outage, oversized message. The sending server retries for 24–72 hours, then converts to a hard bounce. Both affect your bounce rate; only one is your fault.
Hard bounce meaning
A hard bounceis a permanent email delivery failure. The receiving server returned an SMTP response in the 5xx range, signaling that this message cannot be delivered and retrying will not help. The mailbox does not exist, the domain is dead, the recipient has explicitly blocked you, or the message was rejected for policy reasons that won't change.
Common hard bounce causes:
- Invalid recipient (5.1.1): the local part (before the @) does not exist.
- Invalid domain (5.1.2): the domain has no MX record or doesn't exist.
- Blocked sender (5.7.1): the receiving server has explicitly blacklisted you.
- Spam rejection (5.7.x): the message was flagged as spam at SMTP time.
Hard bounces are the most damaging email bounce types because mailbox providers treat them as a list-quality signal. A 5% hard bounce rate on a single send will cost you measurable reputation at Gmail within 24 hours. See cold email bounce rates for the math.
Soft bounce meaning
A soft bounceis a temporary email delivery failure. The receiving server returned an SMTP response in the 4xx range, signaling "not now, try again later". The sending server (your ESP or MTA) will queue the message and retry on a backoff schedule, typically for 24–72 hours, before giving up.
Common soft bounce causes:
- Mailbox full (4.2.2): recipient has exceeded their storage quota.
- Server unavailable (4.4.1): destination server timed out or unreachable.
- Greylisting (4.7.1): server is deferring first-time senders for a few minutes — totally normal.
- Message too large (4.2.3): attachments push past the size limit.
- Rate limited (4.7.0): you're sending too fast for this domain.
Soft bounces are normal. Healthy programs see 1–3% soft bounce rates. They're only a problem when the same address soft-bounces repeatedly — at that point treat it like a hard bounce and suppress.
Comparison table
A side-by-side on the differences between email bounce types:
| Soft bounce | Hard bounce | |
|---|---|---|
| SMTP code | 4xx (transient) | 5xx (permanent) |
| Cause | Mailbox full, server down, oversized, greylisting | Invalid address, dead domain, blacklist, policy reject |
| Retry policy | Yes — 24–72 hours, exponential backoff | No — final failure |
| Suppress? | After 3 consecutive on same campaign | Immediately, every time |
| Reputation impact | Low | High |
| Your fault? | Usually not | Usually yes — bad list |
Bounce code: SMTP 4xx vs 5xx
Every bounce comes with a bounce code — an SMTP status code that tells you exactly what went wrong. The codes follow RFC 3463 in the format X.Y.Z (e.g., 5.1.1).
- First digit (class): 2 = success, 4 = transient failure (soft bounce), 5 = permanent failure (hard bounce).
- Second digit (subject): 0 = unspecified, 1 = addressing, 2 = mailbox, 3 = mail system, 4 = network/routing, 5 = mail delivery protocol, 7 = security/policy.
- Third digit (detail): the specific cause within that subject.
Examples worth memorizing:
5.1.1 Bad destination mailbox → hard bounce, suppress 5.1.2 Bad destination system → hard bounce, suppress 5.7.1 Delivery not authorized → hard bounce, often blacklist 5.7.26 Multiple authentication failed → SPF/DKIM/DMARC problem 4.2.2 Mailbox full → soft bounce, retry 4.4.1 No answer from host → soft bounce, retry 4.7.0 Temporary auth failure → soft bounce, retry 4.7.1 Greylisted → soft bounce, retry
If you see a flood of 5.7.x codes, you have an authentication or reputation problem, not a list problem. Pause, read our email deliverability guide, and audit before resuming.
How bounces affect bounce rate email metrics
Bounce rate email tracking usually combines soft and hard bounces into a single number, which is lazy. They mean different things and need separate thresholds.
Gmail starts spam-foldering at a hard bounce rate of roughly 5% per send. Outlook is even less forgiving — 3%. Healthy programs run under 2%.
Track them separately in your ESP or pipeline:
hard_bounce_rate = hard_bounces / attempted_sends soft_bounce_rate = soft_bounces / attempted_sends combined = (hard + soft) / attempted_sends
What to do about each
Hard bounces:
- Add the address to a permanent suppression list. Never send to it again.
- If >3% on a single send, pause the campaign and revalidate the list.
- If the bounce code is 5.7.x, check Spamhaus / Talos for a blacklist listing.
- Check your sender score and Postmaster reputation.
Soft bounces:
- Let your ESP retry. Don't resend manually.
- If the same address soft-bounces 3 campaigns in a row, suppress it.
- If 4.7.x dominates, slow your send rate per recipient domain.
- If 4.2.2 dominates one domain, that domain may have raised storage limits — wait a week.
Preventing bounces in the first place
The cheapest bounce is the one that never happens. Three habits keep you out of trouble:
- Validate before send. Run every new list through a verifier. Drop catch-all and risk-score >5%.
- Maintain hygiene. Suppress hard bounces immediately. Re-validate any list older than 90 days.
- Warm the domain. A warmed sender with strong engagement tolerates a higher bounce rate before getting throttled. Template-based warmup applies that signal to your actual campaign body.
The 2026 baseline: validate, suppress, warm. In that order.
Frequently asked questions
What is the difference between soft bounce and hard bounce?
A hard bounce is a permanent delivery failure — the address does not exist, the domain is dead, or the recipient explicitly blocked you. A soft bounce is temporary — the mailbox is full, the server is down, or the message was rejected for being too large. Hard bounces should be suppressed immediately; soft bounces are retried by the sending server, usually for 24–72 hours.
What does a hard bounce mean?
A hard bounce means the receiving server returned an SMTP 5xx response code, which is a permanent failure. The most common cause is a nonexistent mailbox (5.1.1) or a nonexistent domain (5.1.2). Hard bounces tell you the address will never accept mail at this domain. Keep sending to it and your sender reputation drops fast.
What does a soft bounce mean?
A soft bounce means the receiving server returned an SMTP 4xx response code — a temporary failure. The mailbox might be full, the server might be overloaded, or the message might be temporarily rejected by greylisting. Your ESP will typically retry the message for 24–72 hours before giving up and converting the soft bounce to a hard bounce.
What is a good bounce rate for email?
For warm, opted-in lists: under 2% combined bounce rate. For cold outreach: under 5% per send is acceptable, under 3% is good. If you hit 10% on a single send, pause the campaign — Gmail and Outlook treat that as a strong list-quality signal and will throttle or spam-folder you within hours.
How do I read a bounce code?
Bounce codes follow RFC 3463 — the format is X.Y.Z. The first digit is class: 2 (success), 4 (transient/soft), 5 (permanent/hard). The second and third digits describe the specific reason: 5.1.1 means the recipient address is invalid; 4.2.2 means mailbox over quota; 5.7.1 means message rejected for policy reasons (often spam filtering).
Do soft bounces count as hard bounces eventually?
Yes, after retry exhaustion. Most ESPs retry a soft bounce for 24–72 hours. If the address still rejects, the message is converted to a hard bounce and the recipient is suppressed. Some platforms also auto-suppress addresses that soft-bounce on three consecutive campaigns, treating chronic soft bounces as effectively dead.
Can warmup reduce bounce rate?
Warmup does not fix bounces — bounces are a list quality problem, not a reputation problem. What warmup does is build the engagement signal that lets your messages survive a higher complaint or bounce rate without getting spam-foldered. Validate the list first, then warm. In that order.
Keep reading
All posts ↗- Sender Score Explained: What It Is, How to Improve ItSender Score is a 0-100 IP reputation number from Validity. Here is what it actually measures, when it matters, when it does not, and how to improve it.
- Domain Reputation: How Mailbox Providers Score Your DomainDomain reputation — what mailbox providers measure, where to check your score (Postmaster, SNDS, Sender Score), and how to repair a damaged sending domain.
- Cold Email vs Marketing Email: The Real DifferencesCold email vs marketing email — different intent, different infrastructure, different rules, and why mixing them on one domain kills deliverability.
- Email Warmup in 2026: The Complete GuideThe 2026 email warmup guide — what warmup actually does, how mailbox providers measure it, why sender-only warmup falls short, and the playbook that works now.