Which operating system? Which version of Outlook? Installed the latest Service Pack for your version of Office/Outlook? (If not, please do.) Is Outlook using POP3 or Microsoft Exchange to receive the mail? Is your version of Office genuine?
Have you made sure you've got the right settings? In particular, make sure the "My outgoing server requires authentication" is CHECKED if you're using POP3 on most ISPs. (In Office 2003m, Open Outlook, click on "Tools-Email Accounts-View or change existing email accounts-"Change" button-"More Settings" button-Outgoing Server tab.)
Hope this helps and let us know more.
MS Outlook error when sending e-mail. Error 0x80070057. How to repair.