To enable SMTP Authentication you must make changes to the Properties of the Account that is trying to send email via Icewarp

Different version of MS outlook have different methods of accessing the Account Properties dialog, so this FAQ is generalized and you should consult the documentation for your version of Outlook to

Once you have located the Account properties dialog for the account that is sending via Icewarp you need to locate the checkbox that tells Outlook to use SMTP authentication and check it.

Here is an example from MS Outlook 2003

You should also select the Use same settings as my incoming mail server