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.
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 behaviour 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.
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