It can force it on the outbound. https://support.symantec.com/us/en/article.tech164655.html
It also allow SIMME pass-through. https://support.symantec.com/us/en/article.tech166867.html

The base64 for the version is not needed. I rebuilt and did a test for that. I was testing with Outlook 2016 to Outlook.com to another exchange server. One of the servers in the chain is converting the mime parts to base64.
The MAPI headers in gpgol are causing the auto-decryption of Symantec to stop checking for the MIME attachments. On internal emails the MAPI format is retained and that causes an issue with the symantec client. When they leave the exchange server the base MIME format is what is sent and that works with the Symantec client.