Bounce rate

Bounce rate = bounced emails ÷ emails sent, shown as a percentage. Most sending tools split it into hard bounces (permanent failures, such as a mailbox that doesn't exist) and soft bounces (temporary failures, such as a full mailbox or a server that's deferring mail).

Bounces arrive in two ways: synchronously, as a rejection during the SMTP conversation, or asynchronously, as a bounce message (non-delivery report) after the receiving system first accepted the mail. Catch-all domains often produce the second kind. Tools differ in which they count and when, so compare like with like.

A high bounce rate tells mailbox providers your list is old, guessed or bought, and it damages your sender reputation. Google's sender guidelines tell senders to reduce volume when messages start bouncing and to unsubscribe recipients with multiple bounces. In cold outreach, bounces usually come from stale data, guessed email permutations and unverified catch-all addresses.

Verifying before you send is the most direct way to bring bounce rate down. Skrap.ai marks addresses that don't exist as Invalid (free), flags catch-all addresses as Risky, and reports Unknown (free) where the server won't answer. The email finder verifies every email before returning it. Start with the free email verifier and read the email bounce rate guide.