Could not load file or assembly

Question

I created a web part that retrieves emails and To-do list from a mailbox using Exchange's web services. The code works great on my UAT server.

 

But when I deployed the solution to my production Server, I'm getting the following error in my ULS logs.

Could not load file or assembly 'Microsoft.Exchange.WebServices, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

 

 I've checked the web.config files of my UAT sites as well as the one located at C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\TEMPLATE\LAYOUTS\web.config and none contain any references to the the Exchange WebServices that I can find.

 

I'm not sure how to get SharePoint to see the assembly.