In Outlook:

1. On the Tools menu, click E-mail Accounts.
2. Select View or change existing e-mail accounts, and then click Next.
3. Select the name of the e-mail account, and then click Change.
4. Click More Settings.
5. On the Outgoing Server tab, verify that the My outgoing server (SMTP) requires authentication check box is selected.

In Outlook Express:

1. From the Tools menu select Accounts. This will bring up the Accounts window
2. Select the email account you wish to enable SMTP authentication for and click on the Properties button
3. Click the Servers Tab
4. Select the My server requires authentication checkbox
5. Click the Settings button
6. Ensure that the Log on Information is set to Use same settings as my incoming mail server
7. Click OK for all windows that are currently open

In Thunderbird:

1. Go to "Tools -> Account Settings -> Outgoing Server (SMTP)".
2. Select the server and press the Edit... button.
3. Check the "Use name and password" option.
4. Add your username.
5. Click "OK".

In Apple Mail

1. From the Mail menu choose Preferences
2. On the lower right corner of the window, click on Server Settings... (under Outgoing Mail Server (SMTP)). If this option is not visible, make sure you have the correct mail account selected, and that the Accounts and Account Information tabs are selected.
3. Pull down the Authentication menu and select Password.
4. Enter your username and password.
5. Click OK.