How can I store 15 years of older MacOS Mail on an external drive?
There are a number of existing questions/answers that address this very topic:


Archiving mail from Mail.app
Archiving Apple Mail to an External Drive
Move mail boxes data to external drive on Mac Mail on Mavericks
Moving individual Mailbox to external drive


These posts should get you on the right track to migrating your mail over to the external disk.

Basically, what you need to do is either copy/move the mbox file to the external drive or use the Export function in the Mail app to export mail to a mailbox located on the external drive.