Also known as: email bounce rate, hard bounce, soft bounce
Bounce rate is the percentage of emails you send that can't be delivered and are returned by the receiving server. A hard bounce is a permanent failure (the address doesn't exist); a soft bounce is temporary (a full mailbox or a server issue). In cold email, bounce rate is one of the clearest signals of list health — a high rate means your list is stale or unverified, and it actively damages your sender reputation.
Keep it under roughly 2–3%. Above about 5% is a red flag, and high bounce rates do real harm beyond the wasted sends: mailbox providers read them as a sign you're emailing a list you don't have permission to use — classic spammer behavior — and start filtering you. Because bounces hurt the reputation that protects every future send, a spike in bounces is a reason to stop and fix the list before sending another batch.
The fix is list hygiene: verify email addresses before sending so invalid mailboxes are removed, keep your list fresh rather than reusing old data, and suppress addresses that have hard-bounced before so you never hit them twice. Verification and ongoing hygiene are the difference between a list that protects your deliverability and one that quietly torches it.
Bounce rate is an early-warning gauge for both list quality and deliverability. Watching it — and acting fast when it climbs — prevents a stale list from dragging down your sender reputation and your inbox placement.
Under about 2–3%. Above roughly 5% is a red flag that your list is stale or unverified, and it damages your sender reputation because providers read high bounces as spammer behavior. If bounces spike, fix the list before sending more.
A hard bounce is a permanent failure — the address doesn't exist — and should be suppressed immediately. A soft bounce is temporary, like a full mailbox or a server hiccup, and may resolve on its own.
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