Enable server authentication within your mail client:

Outlook 2000

1. Go to Tools > Services (or Tools > Accounts, if Services is not listed)
2. Select your Internet E-Mail Account and press “Properties.” Go to the “Servers” tab, and check “My server requires authentication” under Outgoing Mail Server
3. Press “Settings,”
4. Select “Use same settings as my incoming mail server,” then press “OK” three times to save the changes & return to Outlook
   

Outlook 2002 & 2003

1. Go to Tools > E-Mail Accounts
2. Select “View or change existing e-mail accounts” and press “Next”
3. Select your fused POP/SMTP account and press “Change”
4. Press “More Settings”
5. Go to the “Outgoing Server” tab, and check “My outgoing server (SMTP) requires authentication”
6. Select “Use same settings as my incoming mail server” and press “OK”
7. Press “Next,” then press “Finish” to confirm your changes and return to Outlook 

Outlook Express

1. Go to Tools > Accounts
2. Select the “Mail” tab
3. Select your fused account and press “Properties”
4. Go to the “Servers” tab, and check “My server requires authentication” under Outgoing Mail Server
5. Press “Settings,” select “Use same settings as my incoming mail server"
6. Press “OK” twice to save the changes
7. Press “Close” to return to Outlook Express