A soft bounce is a temporary email delivery failure — the recipient's email server has rejected the message but the issue may resolve on retry. Soft bounces are typically caused by transient conditions like a full mailbox, a server outage, or temporary spam-filter rejection. They're distinct from hard bounces, which are permanent failures requiring immediate suppression of the address.

Common causes of soft bounces

Mailbox full: the recipient's inbox storage is at capacity. The address is valid, but messages can't deliver until the recipient frees space.
Recipient server unavailable: the email server is down for maintenance, overloaded, or has a configuration problem. Almost always resolves within hours or days.
Message size limit exceeded: the message (often with attachments or large embedded images) exceeds the recipient server's size threshold.
Temporary greylisting: the recipient server has temporarily deferred unfamiliar senders to validate they're not spam-bots. Resolves automatically on retry.
Out-of-office or auto-reply with delivery deferral: some auto-responder systems return temporary failure codes.
Soft-bounce spam filter rejection: the message scored high enough on spam filters to be deferred but not permanently rejected.

What soft bounces cost in email revenue

Bounce behavior matters because email is usually the highest-margin revenue line a brand has — the marginal send costs almost nothing — and that only holds while the messages reach the inbox. Soft bounces are the early warning that placement is slipping. An isolated one is a full mailbox and means nothing. A rising rate, or a spike concentrated at a single mailbox provider, means the sending domain is being deferred rather than delivered, and by the time that shows up as falling email revenue the reputation takes weeks of reduced, cleaner sending to repair.

The decision it drives is unglamorous and financial: sunset addresses that soft-bounce repeatedly instead of mailing them again, and resist the pressure to send to the whole list during a promotion. The extra sends to dormant addresses buy a little short-term revenue at the cost of deliverability for the buyers who were going to open anyway.

How ESPs handle soft bounces

Email service providers (Klaviyo, Mailchimp, Attentive) typically retry soft-bounced messages automatically over several hours to days before deciding whether to suppress the address. Specifics vary:

Initial soft bounces: retried multiple times, with increasing intervals between retries.
Persistent soft bounces: after typically 4–7 consecutive soft bounces, most ESPs convert the address to a suppressed state, treating it like a hard bounce.
Engagement-aware handling: some ESPs adjust retry behavior based on the address's engagement history. Highly-engaged recipients get more retries before suppression than dormant ones.

Why soft bounce rates matter

Individual soft bounces aren't deliverability problems; persistent soft bounce patterns are. High soft bounce rates can indicate:

List staleness: dormant addresses where mailboxes have filled up because recipients abandoned them.
Reputation issues: the sender's IP or domain is being deferred (not yet rejected) by spam filters as a borderline-suspect sender.
Volume issues: sending volume spikes can trigger temporary throttling that registers as soft bounces across many recipients.

What to monitor

Soft bounce rate per campaign: over 5% indicates deliverability problems worth investigating.
Repeat soft bouncers: addresses that soft-bounce on multiple consecutive sends should be sunset proactively rather than waiting for ESP suppression.
Patterns by ISP: a soft bounce spike from a single domain (gmail.com, outlook.com) usually points to a reputation or content issue specific to that mailbox provider.

Segmenting repeat soft bouncers out of regular sends before an ESP suppresses them automatically is flow logic, not a one-time list cleanup — it has to run on every send, which is the kind of ongoing hygiene that's easiest to build once into the flow and segment architecture of a Klaviyo email and SMS marketing program rather than re-running manually before each campaign. Since a meaningful share of soft bounces come from spam-filter deferrals rather than full mailboxes, heavy image weight and code bloat in a template can quietly push a sender toward that threshold, which is one more reason lean, well-coded templates from a Klaviyo template design process matter beyond how they look in the inbox.

Some soft bounces trace back to authentication rather than list hygiene: a sending domain missing proper SPF and DKIM records can get deferred by mailbox providers even when the recipient address itself is perfectly valid, which is what this guide to configuring SPF and DKIM records on Shopify walks through.