In order to protect your send domain reputation and stop your emails ending up in your customer's spam folder, we enforce a 5% bounce back threshold that keeps your good name clean and your emails delivered.
Campaigns exceeding a 5% bounce back rate (based on the % of emails sent that are delivered to invalid email addresses) will get halted and marked with a campaign status of "failed". This might seem harsh, but the reality is that if we continue sending out that campaign, ESP's will start flagging your send domain as "spammy" and your future campaigns, even the ones with low bounce rates, risk being spammed as the damage has already been done to your reputation.
We send out your emails in batches of 100. If the 1st batch scores less than 5% bounce rate, we proceed to the 2nd batch, and so on. Bounce rate is calculated on the total number of emails sent, so if your first batch scores 0% bounce rate (zero email hard bounced out of the 100 sent), and your 2nd batch scores 6% bounce rate, your total bounce rate would still only be 3% (6/200 x100).