Why am I unable to send an email order using the Ubercart and Smtp Authentification module?
Okay I found the solution using the instructions at "Drupal – Ubercart and SMTP Authentication Support".


I installed Mail System module.
In the Mail System configuration, I updated the Cart module class, Order module class and Store module class so that they all use the SmtpMailSystem rather than UbercartMailSystem, as shown in the screenprint below: