This usually is a result of the mail server requiring authentication.
If you are using Outlook 2003, here are the steps to enable authentication:
Go to Tools | Email Accounts
"View or change existing e-mail accounts" should already be marked
Select the Account with the trouble and click Change
Click on More Settings
Click on the Outgoing Server Tab
Click "My outgoing server (SMTP) requires authentication" and make sure "Use same settings as my incoming mail server" is selected
Click on the Advanced Tab
Set "Outgoing server (SMTP):" port to 26
Click OK and then Test Account Settings
If everything is fine and reports no errors,
If an error does occur, it will switch to the Errors tab and display the error
If you are still having errors, make sure your login username is your full email address and that the password is correct.
If both of these are correct, please, do contact us and we'll investigate your issue further