It maybe that your ISP SMTP server requires authentication.
In Outlook (not Outlook Express) try the following:
Click "Tools" then select E-mail Accounts
Select "View or change existing email accounts and click "Next".
Select your email account and click the "Change" button on the right.
Click the "More Settings..." button in the bottom right-hand corner of the Email Accounts window.
In the Internet E-mail Settings window, click the "Outgoing Server" tab. Ensure that the box is checked next to "My outgoing server (SMTP) requires authentication".
Make sure that you do not tick the box 'Log on using Secure Password Authentication' SPA and also check that on the Advanced Tab that 'This Server requires a secure connection' SSL.
For Outlook Express:
Click, Tools, Accounts.
Highlight, your email account.
On Server Tab, Under Outgoing Mail Server, Tick box 'My server requires authentication'
As Outlook don't tick 'Secure Password Authentication' SPA