As per Microsoft a Non-read Receipt is a message that is generated when an email message is deleted at the expiration of a time limit or due to other client-specific criteria.
The original message that corresponds to the Non-read Receipt was sent with the "Request a Read Receipt" check box. If you open the users mailbox in Outlook and locate the original message and check the properties, you will notice Request a Read Receipt is checked off.
There are several articles/forums around the internet as to why it occurred, it may be something to do with the Outlook client setting it self, an expiration setting, or an Exchange setting. The recipient of the original message also may have possibly opened the message offline and the receipt was not delivered.
There is no concern in regards to Archive Manager. The Non-Read Receipt was generated by Exchange/Outlook, when it arrived in the users Exchange mailbox, Archive Manager archived it.
