Unable to update Mail over wi-fi iPhone

Help! My Yahoo Mail accounts have stopped working over my home wi-fi network. All other apps etc work fine over the wi-fi, it only seems to be my Mail accounts that have a problem.
I have deleted and re-entered the Mail account details, I have told my iPhone to forget my wi-fi network then reinstalled it, Ive updated the phones OS to the latest version available(4.3.1)....and now Im out of ideas.
Anyone?
iPhone 3GS iOS 4