This doesn't sound tehnically viable to route internal mails through smarthost, whatever the software is, and the only alternative I can think of is to use some sort of blind-carbon-copy that automatically copies all mails sent internally to an external address.
I have found this article from SpamExperts Knowledgebase, that deals with the same issue, (although it's with Exchange):
Microsoft Exchange internal email archiving
I'm going to quote it here:
When using SpamExperts for both your inbound and outbound email
filtering (using the smarthost setup), all external SMTP communication
will automatically be archived as part of the domain for which
archiving is enabled. However Microsoft Exchange does not relay
internal communication via the outgoing smarthost, therefore internal
communication will not be archived by default.
Archiving internal communication is simple however, and can be
accomplished with the Exchange journaling system. The journaling
system allows Exchange to automatically send a copy of all internal
communication to an external email address. As long as you've setup
the external email address with SpamExperts for archiving, the
SpamExperts inbound filter will simply process the message and
archives it. You should configure the destination address to which the
journaling reports are sent as a whitelisted blackholed recipient.
This means for the messages received, no filtering or delivery to an
external server takes place. SpamExperts support can help you
accomplish this (or you can do so directly via the Software API on our
Local Cloud product).
Your Microsoft Exchange administrator will be able to activate
journaling for you, to ensure a copy of each email is automatically
sent to the archived blackholed recipient.
Similar things can be achieved by using Postfix's
recipient_bcc_maps options. You may have a look at this for a hint:
How can I configure Postfix to retain copies of all email sent through it?