Exchange 5.5 NT4.0

Recently,I cannot send any email to home, with my
roadrunner account, But I can sent it to by Yahoo or Hotmail, I check with the ISP and they are not blocking out the roadrunner account, Everyone at work with road runner account cannot email homeeither, Is their a setting in exchange that I need to set to get this to wotk.
Thanks in advance,

Tom Bui