>ExchangeServer sign in Enterprise email Administrator Account
Best practice is to never use Administrator accounts in any application, in Exchange especially it won't work because an Admin account (or any user) only has explicit rights to their own mailbox to access another users mailbox permissions need be granted explicitly either via Add-MailboxPermission or Add-MailboxFolderPermission (regardless of your administration rights)
The other option if you need to access many mailboxes within a particular Exchange Organization is to use Impersonation in EWS see https://learn.microsoft.com/en-us/exchange/client-developer/exchange-web-services/impersonation-and-ews-in-exchange again for this don't use your administer account create an application specific account and then grant that impersonation rights.