Definition
A soft bounce is a delivery failure that might succeed later. The receiving server answers with a 4xx code, or with a 5xx code that platforms treat as temporary, and your sending platform usually retries for a while before giving up.
Typical causes:
- The mailbox is full.
- The receiving server is down, overloaded or timing out.
- The server is rate-limiting you because you sent too much too fast.
- Greylisting: the server deliberately refuses the first attempt from an unknown sender and expects a retry.
- The message is too large.
Why it matters
An occasional soft bounce is normal and fixes itself. A pattern is a warning.
When the same address soft-bounces campaign after campaign, it is often an abandoned mailbox that filled up and will never be read. Most email platforms eventually treat repeated soft bounces like a hard bounce and suppress the address.
When many different addresses soft-bounce at once at one provider, look at yourself. Rate-limit and reputation-related deferrals show up as soft bounces before they turn into blocks. Slow down, check authentication, and review recent complaint levels.
How SimpleCold handles it
Verification also runs into temporary conditions, because it talks to the same mail servers. SimpleCold separates them from permanent failures with the failType field: temporary for conditions that may clear, permanent for ones that will not.
When a mail server is slow, single and batch checks answer within about 12 seconds. If the mailbox check has not finished by then, the result comes back with status processing, and asking again within 30 minutes while it is pending does not use another credit. If the server will not confirm the address at all, the result is Risky with the status no_response, so you know it was not a clean pass.
Soft-bounce-prone addresses from old lists are best checked before a send with the bulk email verifier.