By default, if an email hard bounces for 3 consecutive campaigns, their status is automatically changed to 'held' in your account. The 'held' status means that the receiving server has reported the email address as undeliverable. It is possible for a good address to fall into this status occasionally because of the way the receiving server coded the rejection message and sends it back.
1. Navigate to the Contacts dashboard
2. Search for the email address
3. Click on drop down menu of the Edit button to "Edit in new tab"
4. On the right-hand side of the Contact Profile, change 'Status' to normal
6. Select 'Save'
Learn more about Contacts.