Outlook could not connect to Incoming POP3

I am using Outlook 2003. As of two days ago I had no problem receving my emails. But suddently I get the following message:

Log onto incoming mail server (POP3): Outlook could not connect to the incoming POP3 server because the server did not accept your user name or password.

There is nothing in my even viewer either.
I have changed the user password on my exchange server and then on outlook but I am getting the same message. Could someone help and explain also why this sudden problem. Please also note that I have been running the Exchange server and Outlook accounts for the last 7 years.

Thanks in advance