The reason for this behavior is that the company does not have "Capture Email Replies" feature enabled. When the company does not have this feature enabled, no email responses (including the read receipt) are processed by NetSuite. The replies and the eventual read receipt sent by the recipient are delivered to the sender directly - no need to display the Read Receipts tab on the Message record.
Solution is enabling "Capture Email Replies":
- Navigate to Setup > Company > Enable features
- CRM subtab > check "Capture Email Replies"
- Click Save
By enabling this feature the company will start capturing email replies to all emails that are sent from NetSuite. For example, if a support rep responds to a case record by email and the customer replies to the email message, the response is automatically saved on the case record as well as the customer's record. This feature also works with emails sent from the Messages subtab of relationship records. All replies to the original email are saved on the Messages subtab.