I’m testing on my laptop to make sure the Mail directory is on the bootdrive.
After removing and reinstalling the plugin, I go into preferences to enable the plugin and confirm that it has access to the Mail directories.
Hold the Option key and choose Help > Report bug to start a support ticket.
I think I solved the problem. Not only does Mail not like it when it is not on the boot-drive, it doesn’t like it when alternative paths are used. I’ve been using a synthetic mountpoint /map/bones to maintain consistency between my iMac (where my home directory is /Volume/apfs-ext/bones) and my macbook (where my home directory is /Users/bones).
After reverting the macbook to /Users/bones as path, instead of /map/bones, everything worked. Frustratingly, on both machines the Mail application works find with /map/bones. Could it be a hardcoded dependency in the plugin?
Summarizing:
iMac:/Volume/apfs-ext/bones : plugin does not work
synthetic /map/bones to iMac:/Volume/apfs-ext/bones : plugin does not work
synthetic /map/bones to macbook:/Users/bones : plugin does not work
macbook:/Users/bones : plugin works
Thanks for the follow-up!
Indeed you are doing something quite out of the ordinary here.
I can’t answer on any possible dependencies in the plugin, but @cgrunenberg would be the person who would know.
The plug-in doesn’t have any dependencies, it “only” needs to be installed, enabled & accepted by Mail.