With this command you can enable an automatically forward from internal mailbox to an external mailbox with copy:
Set-Mailbox -Identity “John Travolta” -DeliverToMailboxAndForward $true -ForwardingSMTPAddress “user.one@home.net”
Type this command to check the configuration applied in a mailbox:
Get-Mailbox “user.one@home.net” | Format-List ForwardingSMTPAddress,DeliverToMailboxandForward