MacOS Mail cannot access folders on external drive via symlinks

My 2012 Mac Mini is running High Sierra, and I have copied older mail folders to a partition of an external drive. In the User mail folder, I then placed symlinks. In Finder, the symlinks show the complete hierarchical contents of the folders; but, while the Mail program sees all of the folder structure, it does not display the messages within. Any clues will be appreciated!
Edit 01/28 – Permissions look Ok; wonder about "Sandbox" – can't seem to find about that.
I found these similar questions, also relevant, but older OS versions: Moving individual Mailbox to external drive and How to move all of my mails to external HD and make them appear only when its connected and
Transferring Emails saved on Mac to generic folder and
https://discussions.apple.com/thread/7001081

2/2/2020 - Ideas, anyone?
11/28/2020 – add Move mail boxes data to external drive on Mac Mail on Mavericks to the list

Wow, I just discovered that while this would not work in High Sierra, it does work in Mojave!
And then as I did some further importing, it no longer worked ... sigh ...