I recently acquired a second-hand 21.5 inch, i3 iMac my older iMac (OS X 10.7.5) blew up! It came with Sierra installed and was re-populated using Time Machine (external drive). When I opened Mac Mail, the folders layout was different there was still an Archive folder, but it did not function as a drop-down revealing all the sub-folders into which I had organised my mail, but rather had 3 "On My Mac" (OMM) folders - one of which contained all my old emails, but not in their sub-folders. Unfortunately when I went to delete one of the OMM folders - that didn't appear to have any contents, the entire Archive/OMM/OMM/OMM collection disappeared; despite my best efforts I've been unable to retrieve them.

Thinking that I still needed an Archive folder - there was no indication that this was not the case, I created a new Archive folder and again it appeared with 3 On My Mac subfolders, one of which contained all emails that had been contained in the Inbox, Junk & Trash at the time of the old computers demise. I have since gone through these 'legacy' emails and made sure those that I want to keep, appear in either the current Inbox or Junk mailboxes.

Why are there 3 OMM sub-folders under the Archive folder I created can I safely delete them?