I started with Eudora files and converted them to Mac Mail files. Now I have the Eudora files, which I can easily locate onto an external disk because they're flat files.
But I also have that decade+ of converted mail taking up a huge amount of room on my Mac.
I don't want to delete the Mac Mail files, but I want to move them off of my computer and onto an external disk. They are in mailboxes since the Eudora files were sorted into mailboxes.