Outlook email using Verizon DSL problem sending email

I have gone through settup for email on Microsoft Outlook 2000 to use it for email on Verizon DSL and I am having no problems receiving emails on the account. I am unable to send emails on the account. The system is running Windows XP Pro. I have set the email up several times with the same result. Does anybody have any ideas on why I can receive emails but cannot send emails?