Soft bounce
A temporary delivery failure — the address exists but the message could not be handed over.
A soft bounce is a delivery attempt that failed for a reason that may pass: a full mailbox, a server that is down, a message that exceeded a size limit, or a temporary rate limit at the receiving end.
Sending platforms retry soft bounces on their own over a few hours or days. One is not worth looking at; the same address soft-bouncing across several campaigns is, and most platforms eventually reclassify it.
A sudden rise in soft bounces across one domain usually means that domain has started throttling you, which is a deliverability signal rather than a technical accident.