In this case, to be able to address Exchange server in a different domain other than Windows Coexistence server, two variables are set in
BTAvailability Application Settings on Windows Coexistence IIS manager :
EWS-username: ewsusername\domain
EWS-password: password
Change the username to SMTP address format (ewsusername@domain.com) instead of domain\UserName
Test the connection and confirm it is successful.