Outlook has a nasty habit of screwing up server settings. Usually the easiest way to fix the problem is simply to delete the account, restart Outlook and create the account once again. (You won't lose any existing mail by doing so).
However I'm surprised that you've apparently set up the account using the POP3 settings for it. Most people would opt for using IMAP these days (especially if the account is to be accessed from other devices, such as a mobile phone).