Show original email address when forwarding an email

Set-Mailbox  marketing-ForwardingsmtpAddress external@emailaddress.com-DeliverToMailboxAndForward$True

This all works and the emails get forwarded on to the new mailbox, but if the email comes from an external address, so saybob@bob.com emails the marketing mailbox and that gets forwarded on to the external e-mail address, then it shows in the external mailbox as having come from the marketing mailbox. If that makes sense. BUT If I email the marketing mailbox from an internal address it works fine and it shows in the external email address as having come from me. Any ideas how I change show it as having come from an external e-mail address?

Thanks in advance!