trouble sending out email

I am currently having difficulties sending out emails using Microsoft Outlook.  The following message appears when I send out an email:

"Your email message was unable to be sent because the connection to your mail server was interrupted.  Please open your email client and resend the message from the sent message folder".

At the bottom of the message was a link to a page to find a remedy to my problem.  I read through it and followed the instructions telling me to run live update and ensure Norton Internet Security and Norton Anti-virus is up-to-date.  Running Live Update did not cure my problem. 

When I disable Norton Internet Security (NIS), I am able to send email messages through Outlook.  Obviously, I would like to keep NIS enable.

Any idea will be appreciated.