To understand how this test works, it is important to note that it is only possible to know if an email is a hard bounce with 100% certainty when trying to send an email to it, but this action is exactly what could harm your domain's reputation.
So, every time you launch a campaign, RD Station Marketing takes a random sample from the list (a variable percentage) and sends the campaign first to these contacts in order to evaluate the results. If this first partial send exceeds the reasonable limit of Hard Bounces (3% of your Lead base), your campaign will be suspended.
The goal is to ensure that your campaign is not sent to an even greater number of Hard Bounces, which would further damage the reputation of your domain.
When a campaign is suspended, RD Station Marketing shows you the Leads that have received it, so you can see which contacts made up your sample.
Based on this test and mapping, bounce Leads are automatically disabled in your Lead base by RD Station Marketing. This means that, even if these Leads are selected to receive other email campaigns in the future, they won't be qualified by the engagement tool and delivery to them won't be attempted, in order to protect your domain's reputation. You can choose to segment and delete them in order to keep only valid Leads in your base.