Importing Apple mail produces empty folders

Initial setup
1

I found eM Client today and after spending some time with it decided to migrate from Apple Mail to eM Client. I have a large Archive folder (on my local Mac) in Mail, with ~ 50GB of historical emails. Using File > Import, then selecting Import from: Mail, “imports” but the result is that only the folder structure in Apple Mail’s Archive is imported. That is, there are no actual emails, just the folders.

I’ve tried Import selected folders but that, too, only creates empty folders.

How do I actually import the emails themselves?

2

Is there a size limitation for imports? I’ve tried importing sub folders (which are also quite large) but get the same result—just empty folders are imported.