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.

I have worked in a large organization where if someone left the company and their work was divided between multiple people it is easier to copy all the e-mails for that one client/project etc to the person that is taking over that job. Of course users have to have filed all their e-mails correctly for this to work or you will have to have someone do that filing and then copy the folder(s) in question to the users mail file. We actually had a developer figure out a way to do this and we had a database with this code in it. It took forever for this code to run on one folder. Mind you most of the mail files averaged 3 GB in size. Some were are large as 45 GB. Size policies were not allowed so users did what they wanted.

This is the only reason I have ever come across where this feature is useful. Possibly if you have to split an application into two databases bases upon the folder names.