I assume you mean the Outlook client and not an Outlook account ([email protected]). The POP protocol pulls the messages from the server. The message are removed from the server once they are pulled by POP unless you have it set to leave a copy on the server in Outlook. Generally you can’t sync multiple clients with POP.

Synching depends on the mail server you are using. If it’s Exchange you need to reconfigure your clients to use that in place of POP. You can also try IMAP. Those two protocols will synchronize state with multiple clients. You’ll need to talk with your mail provider to see what you can use.

Before you start changing settings I’d suggest making a backup of your mail because there’s a chance the messages are no longer on the server.