Right, drag and drop from Outlook always yields MSG, a Microsoft container the EML only clients cannot read. EML is the plain MIME message, different animal.

At 12GB with folder structure to preserve, use a PST to EML converter. SysTools PST Converter with the EML output option reads the PST directly, no Outlook needed on the machine, recreates the folder tree as directories on disk and writes one .eml per message with attachments inside the MIME.

The free path is a Thunderbird relay: add an IMAP account both clients can see, upload the mail through Outlook, pull it into Thunderbird, then use ImportExportTools NG to save folders as EML. It works but pushing 12GB through an IMAP server takes days and any sync hiccup leaves you guessing which folders are complete.

Verify with counts either way: total EML files on disk per directory versus item count per folder in Outlook. Contacts and calendar items will not convert to EML since they are not mail, plan a separate export for those, CSV for contacts and ICS for the calendar.