A soft bounce is a temporary delivery failure. The address exists, but the email couldn't be delivered right now: the mailbox is full, the receiving server is busy or slowing down new senders, or the message was too large.
What it looks like
The receiving server answers with a 4xx code, which means "try again later". Sending servers retry automatically for a while before giving up.
When a soft bounce isn't harmless
- Repeated soft bounces from the same address over several weeks usually mean an abandoned mailbox. Treat it like a hard bounce and remove it.
- Many soft bounces at one provider at once often mean that provider is throttling you because of volume or reputation. Slow down.
For cold email
A few soft bounces are normal. A sudden rise is a warning about your sending volume or reputation, not about the list.