A contact categorised as a bad address is the result of too many bounces. Sentori has received sufficient, or specific enough, information from the destination mailserver to identify the email address as not working.
Bad addresses are automatically excluded from your sends.
If you believe that a specific address has been incorrectly categorised as bad, you can unset this flag by searching for the contact (Contacts > Contact Search) and amending their contact profile.
Occasionally with older data you may find that you have a number of bad addresses. In this case, you may consider attempting to contact these people via other channels in order to obtain an updated email address for them. You can then import these people as new contacts.
Sentori's interface does not allow you to bulk unset known bad addresses.