Send Mail problems

For some reason I can not get my email program to send mail, have used Outlook, Thunderbird, and Opera’s mail client. None will send mail. I can recieve mail with no problem, but just not send. I get the following error:

“Sending e-mail failed. Maybe the SMTP server is not set up properly? E-mail queued.”

I can send and recieve mail with no problem when I use webmail. I don’t know what is wrong.