Assume that you run the set-ImapSettings -SuppressReadReceipt $true cmdlet to enable read receipts in a Microsoft Exchange Server 2013 environment. When a recipient reads and then deletes the email in an Internet Message Access Protocol (IMAP) or a Post Office Protocol (POP) client, you receive a read receipt and a not read receipt. The symptom confuses you whether the recipient has read the email.
This issue occurs because the enabled SuppressReadReceipt setting causes neither pending read receipt nor NotRead receipt flags to be cleared from the email when the email is read by a recipient.
To work around this issue, run the Set-ImapSettings -SuppressReadReceipt $false cmdlet.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.