Recently trying to install the Mimecast mimecast synchronization engine on an Server with Exchange 2013 Machine , since Exchange 2013 ,

1. Made sure impersonation was setup https://community.mimecast.com/docs/DOC-1734
2. Made sure OWA was working with the Mailbox

Error exchange mailbox validation failed, code: MailboxUnreacheable

EWS restricts you running API requests locally by default.

Try running this software from another machine