Unabel to send emails

I have Office 2003 on my computer and i use Outlook but i am having problems sending emails this is what is happening

When i send the email

It either tells me that I have sent the emails but it comes back that has a mail administrator and I get a mail system report.

Or it does not send the emails and I have to disable my MacAfee Firewall.
This also happens when i email my ISP

I do have a router on and Windows XP with SP2. |also have Mcaffe Intersnet Security on.

I have tried to send emails to myself and i have been able to send and receive them fine.

Does anyone have any ideas what the problems is and how i can solve it