That's a very tricky one. The only idea I see is to ask the support to activate the BCC feature. It's more intended as a system to log every email for compliance reasons, but it do the trick for you. You will have to set a unique, instance-wide, email address and all emails sent out will be copied to that address. From there, your portal will have to extract the email and upload it to the server OR to leverage the view as web page link in the email to copy the content and archive it.