email bouncing back

recently A friend set up an account on her new pc settings as used previously
and seems that sending email is ok but sometimes when received and the receiver of the email trys to reply the email comes back saying that

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:SMTP error from remote mailer after RCPT



although the settings I.E email address all seem ok so what might be causing this?

o/s is windows XP home using outlook express

just not sure if anyone can shed some light on this
Thanks in advance

Rodger.


p.s this even happens if you go to create new email.