Before I start. MozEML files are not you mail, so if they are the text files you have found, forget about them. In search (ctrl+K), click the show list in the middle as the top of the returned results. In the list view, click the square icon on the end of the headings and select folder. Now the list shows what folder they are in. Recent versions I think save sent to a local folders send if there is an error saving them to the IMAP sent folder, but I though there was a dialog went with that asking if you wanted to save locally. This is because errors saving sent are common. Some from the mail provider also updating the sent folder at the same time. Others for unknown reasons. It was seen as a quick way to mitigate potential data loss until the issue was fully resolved.