Outlook Express
From the Tools menu, choose "Accounts."Select the "Mail" tab.Double-click the email account you need to update. (i.e. mail.yourdomain.com).Select the "Servers" tab.Check the box next to "My Server Requires Authentication."Click "Ok."

Outlook '98 and 2000
From the Tools menu, choose "Accounts."Select email account you need to update. (i.e. mail.yourdomain.com).Click "Properties".Select the "Servers" tab.Check the box next to "My Server Requires Authentication."Click "Ok."

Outlook 2002 and 2003
From the Tools menu, select "Email 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 corner of the E-Mail Accounts window.In the Internet Email Settings window, click the "Outgoing Server" tab.Check the box next to "My outgoing server (SMTP) requires authentication."

Outlook 2007
From the Tools menu, select "Account Settings."Select your Email account and click the "Change" button.Click the "More Settings" button in the bottom-right corner of the E-Mail Accounts window.In the Internet Email Settings window, click the "Outgoing Server" tab.Check that the box next to "My outgoing server (SMTP) requires authentication" is checked and "use same settings as my incoming mail server" is selected.

Outlook 2010
From the File menu, select "Info" and choose "Account Settings."Select your Email account and click the "Change" button.Click the "More Settings..." button in the bottom-right corner of the E-Mail Accounts window.In the Internet Email Settings window, click the "Outgoing Server" tab.Check that the box next to "My outgoing server (SMTP) requires authentication" is checked and "use same settings as my incoming mail server" is selected.Mac Mail for OS X

Open Mac Mail.From the "Mail" menu, choose "Preferences."Click the "Accounts" icon at the top of the window.Next to "Outgoing Mail Server (SMTP):" click on the drop-down menu and go to "Edit SMTP Server List"Check to make sure you have the correct SMTP server selected at the top.Verify that "Authentication" is set to "Password."If the "User Name" and "Password" fields are not set, enter your full email address as the username, and your password.Click "Ok."Close the accounts window by clicking on the red circle in the far upper left hand corner of the "Accounts" window.Mac Mail will ask you if you wish to save your changes, make sure to click the "Save" button.