The read receipt functionality in Netsuite also depends on some of the email settings of the sender and recipient. On the recipient side, the read receipt functionality should be enabled, then only the request to acquire the read receipt will reach the recipient side. This read receipt should be accepted by the recipient. Then only it will sent back to Netsuite and added under the read receipt tab of Email. We also believe that this functionality to send read receipts should be enabled on the sender side email settings.
According to the details, we had collected, This will not be supported in personal mail. It should be either work or school accounts.
To enable this read receipt automatically, we can create a script and deploy it to the message record. we assume that this will work for both manually send and buy and ride automatic emails.
For example: If we only need to enable the Request Read Receipt checkbox for sales orders we can add that as a condition in the script so that when sending an email if the record is sales order then readRequestReceipt will be true.
According to our understanding, Read Receipts in Netsuite need acknowledgment from the Recipient side. Depending on the configuration of the recipient’s email client, the recipient can decide to respond to the read receipt request when the message is viewed.
Reference
Read receipts in Outlook on the web
Make read receipts available to users – Google Workspace Admin Help