To setup Courier to work with SMTP authentication please follow these steps:

Click on Accounts in the folder pane, then right click on the mail account in the summary list and select "Properties". Click on the "Mail Server" tab. In the Outgoing Mail Server section, make sure "Use alternate SMTP login method" is NOT checked.

Click on the "Edit" button next to the server name. Select the desired SMTP server from the list and click the "Edit" button. Check the box for "This server requires authentication". For authentication method we recommend using "User Supplied", and manually entering your Username/Password into the appropriate fields.

Click OK four times to close the windows.