Cannot send email

Suddenly I cannot send email. I can receive and brows the internet. Each mail that I tried to send is rejected by the server (although it looks like its not send at all and goes to my inbox directly after the send button is pressed with the message that its been rejected.) Even my own email address is rejected. I have spoken to my ISP – and he says that nothing is blocked and everything is fine on their side. We have gone through all the settings and everything is fine with my mail settings as well. I have tried outlook 2003 and outlook express.