505 Rejection and Junk E-Mail Reporting Program

A friend at college has been having problems using his Yahoo email
account, and also replying to emails from other Hotmail and Yahoo
email accounts. He reportsd that he keeps getting a message stating:

"Remote host said: 550 Mail rejected by Windows Live Hotmail for
policy reasons. A block has been placed against your IP address
because we have received complaints concerning mail coming from that
IP address. We recommend enrolling in our Junk E-Mail Reporting
Program - JMRP"

He states that "Since I had been successfully sending other e-mails
from the same machine only minutes earlier, I was puzzled. Tonight,
from home, I sent another e-mail reply from my computer there, and got
the same bounce message ... snip ... the problems have both arisen
using my Yahoo e-mail account; could it be that the problem is not
about IP addresses, but a Yahoo cock-up? In any case, what might I do
about it? I know of no reason why anyone should complain about e-mails
from my home machine."

Any suggestions please? Many thanks - Chris B.