Contacts in the "Needs Attention"' stat filter will show the list of contacts who have errors on them. Errors need to be resolved first in order for emails to send out.
To clear the contact errors, click the contact's status and the "Clear Error" option will appear.
Clearing contact errors in bulk
Contact errors can be cleared in bulk by checking the "select all" checkbox and selecting the "Clear Errors" option from the drop-down menu.
Clearing specific contact errors
You can clear specific contact errors by selecting the "Error" contact filter and then the specific error from the drop-down menu.