DSL And PPoE

I had previously set up Lan with Demand Dial to ISP and all internal clients could attach to all sites and yahoo mailboxes.Replaced with sbc Yahoo DSL modem,reconfigured RRAS with NAT.Clients computers are not able to attach to certain sites eg ebayand clients can ping yahoo,open home page but cannot get to their mailboxes.All clients can open msn & go to hotmail,log on and check mail or send out mail.I have not configured any filters on my nat server ie the nat server will allow all ports to pass which is default behaviour.My question is has anyone experienced this issue or is there a resolve.Sbcyahoo help notified me they do not support network access.Any thoughts will be welcome.
Thanks