A soft bounce is a temporary delivery failure. The receiving server couldn’t accept the email right now but might later. Typical reasons include a full mailbox, a server that is temporarily down or overloaded, a message that was too large, or the receiver deliberately slowing you down. Soft bounces usually carry an SMTP status code starting with 4.
Sending systems normally retry soft bounces for a while, often over several hours, before giving up. Many soft bounces resolve themselves: the mailbox gets cleared or the server comes back. If the email still can’t be delivered after the retry period, it is recorded as a soft bounce for that campaign.
Repeated soft bounces to the same address are a different matter. An address that has been “full” for months is usually abandoned, and some abandoned addresses are eventually turned into spam traps. That is why most platforms treat an address as undeliverable after several soft bounces within a period and stop sending to it.
A sudden wave of soft bounces at one mailbox provider often isn’t about the addresses at all. It can mean the provider is throttling you because volume jumped or reputation dipped. Read the bounce messages: phrases like “rate limited”, “try again later” or “temporarily deferred” point to throttling, which calls for slowing down rather than cleaning the list.
A Kochi tour operator sees 900 soft bounces on a 30,000-email campaign, almost all from one provider, with messages saying “too many messages, try again later”. It reduces its hourly sending rate for that provider and the next campaign delivers normally.
MailNesto reports soft bounces separately from hard bounces and treats an address as bounced after three soft bounces within 30 days, so you stop mailing it.