The system detects undeliverable emails in two ways. First we scan emails as they are imported and test for deliverability. After that when emails are sent, we collect data on emails that are not successfully delivered. If an email address fails in either of those processes, we will change the contact and email address to a status of "Undeliverable".
When a contact is labeled as this, you cannot send emails to that contact. If you feel this change was made incorrectly and the email address is indeed correct and able to be used, you can certainly click on the "Subscribe Status" field on the contact and change from "Undeliverable" to "Subscribed".