Definition
Bounce rate is the percentage of emails you sent that came back undelivered. The usual formula is bounced messages divided by sent messages, multiplied by 100. If you send 2,000 emails and 30 bounce, your bounce rate is 1.5%.
Most sending platforms split the number into two parts. A hard bounce is a permanent failure, such as a mailbox that does not exist. A soft bounce is temporary, such as a full inbox or a server that is briefly unavailable. Some tools report only hard bounces as "bounce rate", so check which definition your platform uses before you compare numbers.
Why it matters
Mailbox providers watch how often you try to deliver to addresses that do not exist. A sender who keeps hitting dead mailboxes looks like someone using an old, bought or scraped list, and that hurts sender reputation. Once reputation drops, even your good emails start landing in spam.
There is no official bounce threshold from Gmail, Yahoo or Microsoft. What they publish are spam complaint limits, such as Google's guidance to keep the spam rate reported in Postmaster Tools below 0.3%. The common advice to keep bounces under 2% is an industry rule of thumb, and many cold email teams aim lower. Sequencers and email service providers also set their own limits and can pause a campaign or an account when bounces spike.
A high bounce rate is usually a list problem. The addresses were old, mistyped, guessed, or never real. That makes it one of the easier deliverability numbers to fix, because you can remove most bad addresses before you send.
How SimpleCold handles it
SimpleCold checks addresses before you send, so the bounces never happen. Each address comes back as Deliverable, Risky or Undeliverable with a plain reason. Undeliverable results, such as a missing mailbox, a domain that does not exist or a syntax error, are the ones that would have turned into hard bounces. Remove them, decide how to treat the Risky group, and your bounce rate on the next send drops accordingly.
You can estimate what bounces are costing you with the bounce rate calculator.