For All Exchange Mailboxes in an Organisation
get-Mailbox | Set-MailboxJunkEmailConfiguration -TrustedSendersAndDomains @{Add=’Email Address to be white listed’, ‘Domain to be whitelisted’}
For a Single Mailbox
Set-MailboxJunkEmailConfiguration -Identity “User Email” -TrustedSendersAndDomains @{Add=’Email Address to be white listed’, ‘Domain to be whitelisted’}