A couple of months ago Mailbox had a lot of missing eMails in. Then

discovered there was another mailbox there with the missing eMails. The other day I went to check an old folder and there were only eMails from April till present. I had several years of eMails in this mailbox.

I did a search for Internet Orders mailbox in Mail, but nothing came up. Is there a way to retrieve this mailbox and its contents?