Customers are not being prompted to send a read receipt when they get emails from NetSuite that includes request for a read receipt

Read receipts ensure that recipients have received a message, which is important when sending important information or time—sensitive transactions. With NetSuite, you can include read receipts on all outgoing email messages to individual recipients.When you click an Email button, you will see this check box at the "Message" tab of the New Email Message window.

Reference: Help guide (NetSuite Basics : Working with Email : Sending Email from NetSuite : Requesting Read Receipts)

There are certain email clients that will not prompt the recipients to send a read receipt. Here are the possible scenarios:

• The message is opened using a POP client or a Google Apps Sync client.
• The message is opened using an IMAP client, but your administrator has not configured the feature to send receipts automatically.
• The message is sent to a user through a group mailing list or alias. The message needs to be addressed directly to a user's email address in order to receive a read receipt from that user.

The best way to test is to send an email to an Exchange Hosted account. For Gmail accounts, here is the documentation from the Google Help site:

https://support.google.com/mail/answer/1385059?hl=en