Thank you for contacting Aspose support team.
I have searched the available documentation and found that UniqueUri is unique per exchange store and is case sensitive and may be changed if an item is removed. Aspose.Email does not have control over this UniqueUri, as it is generated by exchange server and ExchangeMessageInfo is filled by the exchange. It is assumed that for each message, the UniqueUri is unique however I could not find any information about uniqueness for all over Exchange mailboxes. You may please visit following site for further information: