PR_INTERNET_CONTENT is a property that was supported on Exchange 2000 (and perhaps 2003) back when we had a separate store for SMTP/MIME data. It’s not expected to work against modern versions of Exchange.
Once a message has arrived in Exchange, there is no "full rfc822" message anymore. It was converted to MAPI and the original was thrown away. You can still get a copy of the original headers in PR_TRANSPORT_MESSAGE_HEADERS, but if you want an EML, your only option is to convert the message using Outlook's MAPI<->MIME conversion API. Or do it by hand. But either way, you're not getting the original message because it no longer exists.
Once a message has arrived in Exchange, there is no "full rfc822" message anymore. It was converted to MAPI and the original was thrown away. You can still get a copy of the original headers in PR_TRANSPORT_MESSAGE_HEADERS, but if you want an EML, your only option is to convert the message using Outlook's MAPI<->MIME conversion API. Or do it by hand. But either way, you're not getting the original message because it no longer exists.