Great idea!
not sure about why you would need to rebuild a mailfile this way, but the idea of transferring entire folders has been posted before: { Link }

On the face of it this seems like a good idea, but after some thought I've decided otherwise and voted no.

Firstly there are some minor issues around the fact that a folder contains references to documents, rather than actual documents. If we can copy a folder, the next step is to copy multiple folders. As the target mail DB won't preserve the document IDs, there's no way to deduplicate, so you'll end up with multiple copies of anything shared between folders.

But this is a minor technical niggle compared to the next issue that springs to mind...

Why do this?

If someone needs the information, forward it to them.

If they need it but want to see thr originals, give them access.

But duplicating the mails around is just a good way to waste storage space on servers and create hassles when legal searches occur.

It's not like giving someone access to your mail isn't easy. It is. And that should be how this is done.
If that can't be done, then the mails should be forwarded instead. That way at least there's an audit trail and it doesn't confuse future legal searches or compliance checks that may happen.