You might need to also copy the mailbox.cfg with those grps to retain the metadata, which means you should move it into a blank folder which already has nothing (or a blank mailbox.cfg) already in it. I believe newer smartermail will pull that information from the email itself when its rebuilding missing entries for mailbox.cfg, I don't think 15 did this.
Matt Petty
Senior Software Developer
SmarterTools Inc.
www.smartertools.com