You've just sent a campaign on Klaviyo and you're seeing "soft bounces" in your statistics. Unlike a hard bounce, it's not a disaster. But it's also not something to ignore. Here's what it really means, and what you should do.
Soft bounce: simple definition
A soft bounce is a temporary delivery failure. The email was not delivered, but not because the address is invalid. The address does exist. The problem comes from something else: the recipient's server was unavailable, their inbox was full, or your email was too large to get through.
Good news: unlike a hard bounce, a soft bounce is not permanent. A new delivery attempt is possible, and the address remains in your list.
What's the difference with a hard bounce?
This is the question we get asked most often. Here's the distinction in one sentence:
A soft bounce is temporary. A hard bounce is permanent.
With a soft bounce, the email address is valid, the problem comes from the timing or the server. We can try again. With a hard bounce, the address is invalid or permanently blocked. We remove it, we don't insist.
Both types of bounces impact your deliverability if they accumulate, but they are not handled in the same way.
What causes a soft bounce?
Several situations can cause a soft bounce.
The recipient's inbox is full. This is the most common cause. The recipient's server can no longer accept new messages. The email bounces temporarily, but if the situation repeats regularly for the same address, it's often a sign that the inbox is no longer really used.
The recipient's server is temporarily unavailable. Technical failures, maintenance, overload: it happens. In this case, the problem is not with you or your email. A new attempt a few hours later may be enough.
The message is too large. Some servers refuse emails that exceed a certain size. If your emails contain heavy images, attachments, or overly dense HTML code, they can trigger a soft bounce on certain mailboxes.
The email content is detected as suspicious. Some anti-spam filters generate soft bounces before deciding whether the email deserves a permanent rejection. This is a serious signal: if your subject lines or content resemble spam, you risk going from a soft bounce to a hard bounce, or even being blacklisted.
How Klaviyo automatically handles soft bounces
On Klaviyo, soft bounces are handled without manual intervention on your part, according to a progressive logic.
After a first soft bounce, Klaviyo automatically retries sending according to its own rules. If the address repeatedly generates soft bounces across multiple campaigns, it is eventually treated as a hard bounce and permanently excluded from your mailings.
This is a smart mechanism that protects your sender reputation without forcing you to manually manage every error. But it doesn't exempt you from regularly monitoring your statistics to identify trends.
Why too many soft bounces are still problematic
One might think: "It's temporary, it will sort itself out." Not always.
A high soft bounce rate over several successive campaigns sends a bad signal to anti-spam filters. Gmail, Orange, SFR, and other email services constantly analyze your sending behavior. Too many errors, even temporary ones, can gradually degrade your sender reputation and cause your next campaigns to land in the "Promotions" tab or directly in spam, even for your most engaged subscribers.
How to reduce soft bounces: 4 concrete actions
1. Reduce the size of your emails. Compress your images, limit unnecessary HTML blocks, avoid attachments. A well-optimized email gets through better on all mailboxes and generates fewer technical errors.
2. Regularly clean your list. Addresses that repeatedly generate soft bounces deserve close monitoring. If an address systematically bounces, it has probably been abandoned by its owner. It's better to delete it than to keep sending to it.
3. Enable double opt-in. By confirming the address upon registration, you limit input errors and ensure that each contact on your list corresponds to a genuinely used mailbox.
4. Work on your content and subject lines. If your soft bounces are linked to anti-spam filters, it's often a sign that your content or subject line contains suspicious elements. Avoid spam trigger words, take care of your SPF, DKIM, and DMARC authentication, and maintain a good text/image ratio in your emails.
Soft bounce vs. hard bounce: the summary table
Soft bounce: temporary error, valid address, new attempt possible, moderate impact on deliverability if occasional.
Hard bounce: permanent error, invalid or blocked address, no new attempt, direct and serious impact on sender reputation.
Both deserve to be monitored. But the soft bounce allows for more leeway than the hard bounce.
What The Modern Letter does to monitor your deliverability
Deliverability is the foundation of any effective email strategy. At The Modern Letter, every support package begins with a complete audit of your Klaviyo account: bounce rate, list quality, authentication configuration, segmentation of engaged profiles.
Because a clean list is the number one condition for your emails to actually reach the inbox. And reaching the inbox is the number one condition for generating revenue.
