Identify the blocked sender domain: https://admin.google.com/ac/emaillogsearch
If the staff member doesn't know which email address to unblock, use Reports > Email Log Search in the Admin Console to try and identify the blocked address. It should say rejected under "Recipient Details".
Add sender to the Migrated Restrict Delivery Domains list: https://admin.google.com/ac/apps/gmail/manageaddresslist?addressListType=1
Use the link to find the Migrated Restrict Delivery Domains address list in the Admin Console. Choose "Edit" and add the domain portion of the email -- after the @ -- to the list of allowed addresses.
