Enable Mailbox Sharing and Delegation using the Add-MSOnlineMailPermission powershell cmdlet. I went over configuring powershell for BPOS in my BPOS in my Email forwarding in Exchange Online BPOS post.

This command grants [email protected] (TrustedUser) full access and send as permissions on [email protected]’s (Identity) mailbox using the $cred credentials to perform the action.

$cred will be your BPOS administration user credentials

$cred = get-credential
Add-MSOnlineMailPermission -Identity [email protected] -Credential $cred -TrustedUser [email protected] –GrantFullAccess True –GrantSendAs True