To enable SMTP Authentication in your email client:

In Outlook or Outlook Express:

Click tools, accounts, mail tab, then double click your mail account.  Click the servers tab, then under "Outgoing mail server, check "my server requires authentication".

In Outlook XP,

Click tools, email accounts, view or change existing email accounts, double click the account, more settings, outgoing server tab, then check the box for "my server requires authentication".

In Netscape:

There are no additional settings to enable SMTP Authentication in Netscape Messenger

In Eudora,

This is checked by default, but just in case, click tools, options, then check "Allow authentication