Bounce rate (hard and soft)

When a receiving server refuses a message, it answers with a code. The SMTP standard divides these into permanent failures, the 5xx replies, after which the sender should not repeat the same request, and temporary failures, the 4xx replies, which may succeed if tried again later. Senders call the first hard bounces and the second soft bounces.

Hard bounces are the ones that hurt. They usually mean the address never existed or has been closed, and a high share tells mailbox providers that the list was not checked. Every hard-bounced address should be suppressed at once and never sent to again. Soft bounces can be retried a few times, but an address that keeps soft-bouncing should be treated like a hard bounce. The best way to keep the bounce rate low is to verify addresses shortly before sending and to skip catch-all domains you cannot verify.

• In Outlevo: A campaign pauses itself above 3% hard bounces or 5% unsubscribes over its last 200 sends, or at a spam complaint.

Sources: RFC 5321 (SMTP), section 4.2.1 (rfc-editor.org, checked oct 22, 2026).