NEW MAIL SETTINGS
Due to the increased incidence of spam in our datacenter network, outgoing mail now requires authentication. This new security precaution may require you to reconfigure a minor change to your settings in your Outlook Client. Web Mail is unaffected by these new settings.

Outlook XP
Tools --> Mail Accounts --> View or change existing email accounts --> Select the Account from the account listings --> Change --> More Settings --> Outgoing Server

Check the checkbox for “My outgoing server(SMTP) requires authentication”

Click OK
Click Next
Click Finish

Outlook Express 6.0
Select Tools
Select Accounts
Click the Mail Tab
Highlight the mail account in question and go to Properties
Click on the Servers tab
Under Outgoing Mail Server (at the bottom), check 'My server requires authentication'.

Click OK
Click Close.