Bounce

A bounce is a delivery failure reported by the receiving server. There are two kinds, and treating them alike is one of the more expensive mistakes in email.

Hard bounce

Permanent. The address does not exist, the domain does not exist, or the server has refused it outright. There is no retry that helps, and sending to it again is a signal to the mailbox provider that you do not maintain your list.

A hard-bounced address must never be mailed again. In Sendly it goes onto your suppression list automatically, so this is enforced rather than left to your discipline.

Soft bounce

Temporary. A full mailbox, a server that is down, a message over a size limit, or throttling because you sent too fast. The right response is to retry later, and to stop retrying after a few failures — an address that soft-bounces for weeks is a hard bounce that has not admitted it yet.

Why the ratio matters

Sender reputation is affected by the rate as much as the count. A high hard-bounce rate is read as a list that was bought, scraped, or left untouched for years, and providers respond by filtering the rest of your mail.

In Sendly

Bounce notifications arrive at the Return-Path, which Sendly monitors. Hard bounces are suppressed automatically. Bounce events also reach your webhooks and appear in the delivery record for the message, so your own systems can react — marking the contact, pausing a workflow, or asking the customer for a new address.

See List hygiene.