There are two ways that one email can be flagged as "bounced email".
email that actually bounced
email that didn't pass our automated email validation
When does an email bounce?
When an email message cannot be delivered to an email address it's called a bounce. There can be various reasons for an email to bounce.
Metrilo's automated email validation checks if a customer email is valid.
Every time an email comes to Metrilo, it goes through a special validation process where our system checks if the email is valid. We use a 3-rd party service for that. If the email is invalid, Metrilo considers it as a bounced email.
Sometimes a valid email can bounce, there are many reasons where an email can bounce like "return to sender" message (or, SMTP reply). When that happens, Metrilo once again flags that email as bounced
Metrilo automatically ignores bounced emails from your email campaigns.
Bounced emails hurt your domain reputation
Bounced emails hurt your domain reputation and can cause a drop in your email deliverability. High email bounce rate can trigger spam filters for the email providers and thus blacklist your email domain for some email providers.
That's why Metrilo has these validations to prevent you from sending emails to email addresses that will bounce.