Hard bounce and soft bounce: definition, causes and solutions

A hard bounce is a permanent rejection of an email: the address doesn’t exist or the domain is invalid, and delivery will fail every time. A soft bounce is a temporary rejection (full inbox, server unavailable) and a retry may succeed. The rule: remove hard bounces from your list as soon as you’re notified, and monitor soft bounces to remove addresses that bounce on 3 consecutive sends.

Telling soft bounces and hard bounces apart

A bounce is a message or email that never reached the recipient’s inbox, and it hurts a campaign’s performance. Whether you’re dealing with a soft bounce or a hard bounce changes what you should do next. The SMTP notifications sent back by the recipient’s server hold the details you need, though the wording can be subtle. Read the code correctly and you can respond before bounces pile up and start dragging down the quality of your list.

What is a hard bounce?

The hard bounce rate flags an email address that doesn’t work. It’s a permanent issue, identified by an SMTP code in the 5xx family (a 550 Permanent Failure, for example), explained by several recurring causes:

Deleted or deactivated address: closed account, employee who left the company, mailbox purged by the ISP. The recipient server replies that the user no longer exists. These addresses must be removed from your lists immediately.
Mistyped address: a typo made during opt-in, or a fake email deliberately entered by the contact. Setting up opt-in or double opt-in with syntax verification reduces this risk at the source.
Blocked by the recipient server: your IP or domain has a poor reputation, your content triggers filters, or your sending frequency exceeds tolerated limits. The recipient’s platform then refuses delivery.
Unconfigured or expired domain: a missing MX record, broken DNS, or an unrenewed domain name systematically blocks messages sent to that address.
Authentication failure: SPF, DKIM, or DMARC misconfigured on the sender’s side leads to an outright rejection by strict servers. Upgrading the protocol with DKIM2 strengthens this signature and reduces false positives.

A hard bounce can also come from a manual block set by the recipient, who added your address to a personal blocklist, or from a misconfigured inbox. Hard bounces don’t resolve themselves. Don’t bother retrying. Remove the address as soon as you get the failure notification, or your failure rate keeps climbing and your sender reputation takes the hit on future campaigns.

What is a soft bounce?

A soft bounce is more forgiving than a hard bounce and leaves you more options. It signals a temporary issue preventing delivery to the recipient’s inbox, and is identified by a 4xx SMTP code (typically a 421 or a 451):

Full inbox: the recipient can no longer receive new messages. Watch out: a saturated inbox often points to an inactive account. If the person never clears their inbox, you might as well set that address aside to protect your engagement metrics.
Mail server down or offline: ISP outage, technical maintenance, temporary overload. The email will typically go through once the issue is resolved on the recipient’s server.
Message too large: your marketing email or message exceeds the size limit set by the recipient’s server. Photos and videos add up fast, so check the size before sending and prefer links to external hosting.
Greylisting or temporary DNS issue: the recipient server applies a defensive delay on new senders to filter out spam. Most email platforms automatically retry the message after a few minutes.
Spam filter triggered: your message was flagged as spam by the recipient’s filter or their ISP. The cause lies in the content, subject line, or your domain’s reputation.

A soft bounce is far less serious than a hard bounce: the issue is temporary and the contact can be recovered. If the bounce persists across several consecutive sends, it’s best to remove the address before it turns into a hard bounce and drags down your failure rate.

Which SMTP codes indicate a hard bounce?

Errors returned by the recipient server fall into two broad code levels, which immediately tell you how to respond:

4xx = soft bounce, temporary issue, retry possible.
5xx = hard bounce, permanent issue, remove immediately.

For the full breakdown of each code and how to respond to it, see our SMTP error codes guide.

Immediate or delayed bounce: how long should you wait?

A hard bounce comes back fast: the receiving server refuses the address during the SMTP transaction and the error message lands within seconds. A soft bounce follows a different clock. Your sending platform retries at regular intervals, usually for 24 to 72 hours, and the final rejection only shows up once those attempts run out. A campaign report can therefore log new bounces 3 days after the send. Wait that long before reading your final bounce rate, or you will underestimate it. Some servers also reject the first attempt from any unknown sender on purpose, an anti-spam mechanism covered in our article on greylisting: the email goes through on the second try, with no action needed on your side.

Soft or hard: when bounces get misclassified

The line between 4xx and 5xx codes is cleaner in theory than in practice. Some servers return a permanent code for a temporary block (strict anti-spam policy, IP reputation issues), while a mailbox that has been full for months keeps producing soft bounces even though the address is dead for all practical purposes. Sending platforms compensate with rules of their own: after several consecutive soft bounces on the same address, most of them reclassify it as a hard bounce. Klaviyo suppresses an address after 7 consecutive soft bounces, whereas Mailchimp allows 7 to 15 depending on the contact’s activity. The same list can show different numbers from one tool to the next. Repetition is the reliable signal: an address that soft-bounces on every campaign should be treated as a hard bounce, whatever the label says. Our guide on undelivered emails covers how to sort these edge cases.

Auto-replies and cached bounces: two traps in your reports

First trap: the auto-reply. An out-of-office message proves the email reached the mailbox, since the mailbox answered. Most tools keep it out of the bounce rate, though some wrongly count it as a soft bounce: the address works, and deleting a contact who is on vacation would be a mistake. Second trap: cached hard bounces. Most sending platforms add every rejected address to an internal suppression list and skip future sends without even contacting the receiving server. An address that has become valid again (a fixed typo, a mailbox recreated after a job change) can stay blocked indefinitely. If a contact swears they receive nothing even though their address checks out, look for it in your platform’s suppression list and remove it manually.

What impact do bounces have on deliverability?

Bounces weigh directly on your sender reputation, and therefore on the deliverability of all your upcoming email campaigns. An overall bounce rate above 2% (hard and soft combined) starts triggering the spam filters of ISPs and mailbox providers. Above 5%, your sending IP enters the red zone: throttling, quarantine, or even a full block.

The thresholds tolerated by major email platforms give a sense of the safety margins:

Mailchimp: automatic warning through its Omnivore system as soon as a campaign exceeds industry bounce standards.
Mailerlite: recommends staying under 2% overall.
Sendgrid: recommends staying under 5% hard bounces; the suspension threshold isn’t officially disclosed.
Mailchimp (benchmarks across all industries): soft bounces between 0.12% and 1.39%, hard bounces between 0.07% and 0.98% depending on the sector.

On the reputation side, every hard bounce feeds a negative signal tracked by ISPs and scoring tools like Sender Score. If your score drops, your emails land in spam more often, your open rates collapse, and your domain can end up on a public blocklist (Spamhaus, Barracuda, Sorbs). Getting off a blocklist can take several weeks and requires filing a request with the list operator, with no guarantee of reinstatement.

Deliverability isn’t decided only at send time. Every unaddressed bounce erodes your sender capital for the next campaign, so monitoring the relevant blocklists and cleaning your lists upfront is the most cost-effective approach you have.

Other causes of bounces

From one mail server to another, a bounce can be judged differently: soft for some, hard for others. That inconsistency complicates the work of email teams, and a few other factors influence bounces indirectly:

Content

Facing the flood of spam hitting the web every day, ISPs step up their vigilance and block many emails to limit phishing. Your message or email can get blocked before it even arrives because of a slip-up in the content. To avoid this, get the text-to-image ratio right and avoid risky words (free, money, urgent, help me). Also work on the email subject line, keeping it short and precise. Limit repeated capital letters and excessive punctuation.

Reputation = deliverability

The deliverability of your emails also depends on your sender reputation, built up over previous sends and the engagement they generated. To protect this reputation and limit hard bounces, avoid spam traps at all costs by regularly cleaning your marketing lists and consistently applying authentication best practices.

The case of purchased lists

Never buy mailing lists. Packed with spam traps and invalid addresses, they’ll cost you far more than they bring in and will mechanically lead to a high bounce rate. Your email campaigns tank, your domain gets burned, and rebuilding your reputation takes months.

How do you monitor and fix your bounces?

Reacting to a single bounce isn’t enough. Monitoring needs to be ongoing, across four metrics you should track for every campaign and your overall sending activity:

Hard bounce rate per campaign: alert threshold 2%, immediate action beyond that.
Recurring soft bounce rate: if the same address soft-bounces 3 sends in a row, move it to removal.
Weekly trend of the overall rate: an upward drift signals an aging list or authentication going wrong.
Reputation score: your Sender Score and reports from Postmaster tools (Google, Microsoft) give the real picture from the ISP’s side.

Prevention comes down to verifying emails ahead of every campaign. A validation tool like CaptainVerify, or one of the market’s email verification tools, filters out anything that would bounce the campaign before you send: invalid addresses, syntax errors, known spam traps, dead domains. That’s what keeps your bounce rate safely under the critical threshold. A clean database is the most profitable asset of a serious email marketing strategy.

Email marketing best practices: checklist

Check and clean your list regularly with a tool like CaptainVerify before every major campaign.
Set up double opt-in to block typos and fake emails right at collection.
Size your campaigns carefully, avoiding overly heavy files that trigger soft bounces on the recipient’s server.
Monitor soft bounces and remove the address after 3 failed attempts to prevent it from turning into a hard bounce and dragging down your failure rate.
Remove hard bounces immediately. Don’t retry them, and don’t give them a second chance.
Keep your overall bounce rate under 2% to protect your sending IP and domain.
Authenticate your domain with up-to-date SPF, DKIM, and DMARC to avoid protocol-related hard bounces.

To cut bounces even further, check your mailbox provider’s reputation: if it’s poor, you’ll suffer for it too. You can earn Sender Score certification and find the right sending frequency so recipients recognize your messages without feeling overwhelmed.

Deliverability isn’t just about the percentage of emails landing in the inbox. Protecting your sender reputation starts with how you handle the very first bounce, whether that means excluding the address, adjusting your sending habits, or re-engaging the contact instead.

If your latest campaigns are showing hard bounces, run your file through CaptainVerify’s email list verification tool: the analysis flags invalid and risky addresses in minutes, so your next send starts on a clean base.