In an Exchange Server 2016 environment, a sender attaches a read receipt to a message and sends it to an IMAP user. If the message is not read and is deleted by the recipient, an "unread" notification is returned to the sender. This behavior occurs even if the -SuppressReadReceipt option is set to $True through the Set-IMAPSetting cmdlet.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about the terminology that Microsoft uses to describe software updates.