It could also be that your Exchange administrator disabled POP3 by mistake. Or, if you upgraded to Exchange 2010 recently, POP3 is off by default and needs to be turned back.
There's also the possiblility that Plain Text Authentication was turned off by mistake.
Your HelpDesk has to match Exchange exactly. There are some similar authentications available to Exchange, but the Exchange Account must match the HelpDesk exactly.
I went through this when we upgraded to Exchange 2010. Our admin didn't want to enable Plain Text Authentication, but he was finally convinced otherwise.