I found the error at queue viewer tools after coexistence with Exchange 2013 and Exchange 2019. The problem occur when I try to open the queue viewer tools at Exchange 2013 hub transport server.( Please see as attach file) The error message is Microsoft Exchange Critical Error Could not load file or assembly Microsoft Exchange.Data.Transport, Version=15.2.854.0, Culture-neutral, PublicKey Token=31bf3856ad364e35 or one of it's dependencies. The system cannot find the file specified. I installed Exchange 2019 for migration mailbox from Exchange 2013 to Exchange 2019. My Exchange environment. 1. CAS02 Role CAS/HUB Exchange 2013 Std, Windows 2012 R2. 2. BOX02 Role Mailbox Exchange 2013 Std, Windows 2012 R2. 3. MAIL Role Exchange 2019, Windows 2019 Std. Try to open Queue viewer at Exchange 2019 that no any error. My question is. 1. We can not open message queue viewer on exchange 2013 after coexistence with exchange 2019 right? 2. If no, How to fix my problem?