HELP!! can't get email: ERR Service Unavailable * error occurred on authentication * Authentication failed.

I installed a new email client. Possibly it downloaded all the old email and cause a problem.

But now suddenly I cannot download any email in the old computer. I have to use webmail which is still functioning.

Can someone help fix this?

POP< -ERR Service Unavailable

*** error occurred on authentication

*** Authentication failed.

HELP!! can't get email: ERR Service Unavailable *** error occurred on authentication *** Authentication failed.