Hi
I had/have the latest version of the mail activity package. However, there were updates for the System and UIAutomation activities, so I updated those, just to be sure. Unfortunately, I still get the same error 
I’ve no idea how to make sure that the Outlook Users are configured well. But in my case Outlook is not asking for my credentials, so I suppose it’s OK, especially since I had no troubles with Practice 1 (moving emails from one folder to another).
So, here’s some more information:
About the bug:
Scenario: see walkthrough of Level 1 - Foundation Training - revamped - Lesson 11 - Practice 2
Steps to reproduce: Send Outlook Mail Message, with subject = "test" and body = "test", no attachments (yet)
Expected Behavior: Email gets sent
Current Behavior: Email gets NOT sent, error message appears

Version of the software with the bug:
Studio/Robot/Orchestrator Version:
Studio 2018.4.0 - 11/22/2018
Community Edition
EXE Installer
Expires in 10 weeks
License Provider: External
Device ID: dXuIfX55QFg3kAEDHQLl
Update Channel: Stable
Microsoft Windows 7 Enterprise 64-bit
.NET Framework Version 4.7.2 or later
Activity Package Version (when relevant):

Sample process (packaged as a zip file):
Lesson_11.zip (1.5 MB)
(Practice_2.xaml is the one with the error)
Last stable behaviour: n/a
Last stable version: n/a
Versions of the software relevant to the bug:
OS Version:

Others if Relevant: (workflow, logs, .net version, service pack, etc):

Other info that might be relevant:
I had no problems with Lesson 11 Practice 1 (moving emails to another folder)
Thank you for investigating this and I will gladly give more information if necessary.