What Is a Soft Bounce?
TL;DR
A soft bounce is a temporary delivery failure, returned as a 4yz SMTP reply, which means the message might be accepted on a later attempt. A full mailbox, a receiving server that is down, greylisting and rate limiting all produce one, so the sending server keeps the message queued and retries on a schedule.
What causes a soft bounce?
Four common cases, none of which say the address is wrong: the mailbox is over quota, the receiving server is unreachable or overloaded, the receiver is greylisting first contact, or the receiver is rate limiting your sending. RFC 5321 classifies all of these as transient, meaning the same address might be accepted if the client tries again later.
A fifth case hides among them. Some receivers use a 4yz reply as a soft refusal for reputation reasons, so a soft bounce that repeats at one provider while everything else delivers is a filtering signal rather than a mailbox problem.
When does a soft bounce become a hard bounce?
When the sending server gives up. Mail servers retry a transient failure over a bounded window, and once that window expires they generate a permanent failure and return the message to the sender, which is why the same address can appear as a soft bounce for a day and a hard bounce afterwards.
The practical rule is to treat repetition rather than a single event as the signal. One soft bounce needs no action; the same address soft bouncing across several campaigns is worth suppressing, because the mailbox is probably abandoned and full rather than temporarily busy.
Frequently asked questions
Individually no, in volume yes. Receivers expect some transient failures, so a normal rate is background noise. A large share of one send deferring at a single provider is that provider throttling you, and pushing more volume at it is what turns a reputation signal into a reputation problem.