Bounce rate
Bounce rate is the share of sent emails that could not be delivered, shown as a percent of all emails you sent.
Bounce rate is bounced emails divided by emails sent, times 100. A bounce means the receiving server refused the message or could not deliver it. There are two kinds. A hard bounce is a permanent failure, such as an address that does not exist. A soft bounce is a temporary failure, such as a full mailbox.
Bounce rate is one of the clearest signals to providers that you do not know your list. A common target is to stay below 2 percent, and many cold senders aim for under 1 percent. A high rate harms sender reputation fast, and it often leads to a block.
The main cause is a bad list: old data, guessed addresses, scraped addresses and purchased lists. The fix is to verify addresses before you send, remove every hard bounce at once, and test a new list in a small batch first. If your rate jumps, pause the campaign and find the cause before you send more.
In Terlido
Terlido pauses a running campaign when its hard-bounce rate goes above the campaign's limit, after at least 20 sends. The limit is 5 percent by default and you can change it per campaign.