You need to know your account provider's POP server settings, which you can find from the help documentation about POP. For example, to add Gmail as a POP account to Outlook, enable POP access in Gmail from Settings > See all settings > Forwarding and POP/IMAP > Enable POP for all mail. Then, add the account in Outlook > select POP from the Advanced Setup options > and enter Gmail's POP settings to begin syncing mail.