A soft bounce is an email that could not be delivered for a temporary reason, such as a full mailbox, a receiving server that is briefly unavailable, or a message that is too large. Unlike a hard bounce, the address itself is valid, so a later attempt may well succeed.
How a soft bounce works
When your email platform hands a message to the recipient’s mail server, the server replies with a status code. Codes beginning with 4 mean “not now, try later”; codes beginning with 5 mean a permanent refusal. A 4xx response is recorded as a soft bounce.
Common causes include:
- the mailbox is over its storage limit;
- the receiving server is down, overloaded or under maintenance;
- greylisting, where a server deliberately turns away the first attempt from an unfamiliar sender and accepts a retry;
- the message exceeds the recipient’s size limit;
- rate limiting, where a provider slows down mail from a sender it is not yet sure about.
Most platforms retry soft-bounced messages automatically over a period of hours or a few days. If an address keeps soft bouncing across several campaigns, the platform usually reclassifies it as undeliverable and stops sending to it. The number of retries and campaigns before that happens varies between platforms, so check how yours handles it.
Why it matters
A handful of soft bounces in every campaign is normal. A pattern is a warning. Repeated soft bounces from the same addresses usually point to an abandoned mailbox, and abandoned mailboxes are eventually closed or turned into recycled spam traps.
A sudden wave of soft bounces concentrated at one provider means something else: that provider may be deferring your mail because it has doubts about you. The bounce messages often say so, mentioning reputation, unusual volume or policy. That is an early sign of a sender reputation problem, and acting quickly matters.
Business-to-business lists add another case. Addresses at company domains often sit behind strict security gateways, and these can soft bounce repeatedly because the gateway is suspicious of your mail, not because the person has left. If one large client’s domain keeps soft bouncing, it is worth asking your contact there to check.
Common mistakes
- Ignoring soft bounces because they are “temporary”. The same address soft bouncing month after month is not a temporary problem.
- Removing every soft bounce straight away. Deleting an address after one full-mailbox bounce loses a real subscriber.
- Reading only the totals. Grouping bounces by receiving domain shows whether the problem is scattered or concentrated at one provider.
- Re-importing addresses your platform has stopped mailing. If the platform suppressed an address after repeated failures, uploading it again from an old spreadsheet undoes the protection.
How to act on it
After each campaign, open the bounce report and sort soft bounces by domain and by bounce message. Scattered, one-off soft bounces need no action. A cluster at one provider calls for a closer look at your recent complaint rate, any change in volume and your authentication, and possibly a smaller send to your most engaged subscribers at that provider for a while.
If your platform has no rule for persistent soft bounces, set one yourself: for example, move an address to your suppression list after it has soft bounced on several consecutive campaigns with no engagement in between. Review the wider list regularly as part of list hygiene.
When bounces are one symptom among several of email underperforming, a broader digital marketing strategy review is often the quickest route to the cause.