Outlook repeatedly sending same e-mails

Hello,

I have been using Gmail through Outlook Express for years with no troubles. Yesterday, Outlook began showing that e-mails were not being sent, but there was no error message (under the Tasks tab under Status it does say "Warning" and there was also a warning yellow triangle on the task bar.) The message remained in the outbox. But the messages were in fact being sent, dozens of them every time Outlook accessed Gmail. I haven't changed anything. Did a virus scan on Norton with no result. Can anyone help? I'm stumped. Margo