Original title: One of our e-mails cannot send or receive--a box keeps popping up asking for its user name and password--I enter them, but it just keeps popping up, not accepting what I enter--help please

One of our e-mails cannot send or receive--a box keeps popping up asking for its user name and password--I enter them, but it just keeps popping up, not accepting what I enter--help please.

We get e-mail through Outlook 2013 on our Windows8