getting a network logon prompt when replying to e-mails

Hello all,

A good friend of mine works at a local community college and they use Outlook Web client. Now every once in awhile when she sends out e-mails (this happens to various people) the person will get what looks like a network logon prompt when they attempt to reply to her e-mail (they first get a message indicating that you must log onto the college domain).

Any ideas on what is the problem?

Thank you!