Outlook 2016 bug for POP accounts
Recently, we’ve had quite a few customers write in to complain that their copy of Outlook 2016 is behaving incorrectly: it is either deleting messages from the server when it is not supposed to do so, or it is downloading duplicate copies of mail from the server. This happens for POP accounts, not for IMAP accounts (which is what we normally recommend customers to use).
These problems happen because of a bug in Outlook 2016. Microsoft has a Web page that explains the problem as well as the solution (upgrade Outlook).