Behavior:
The mail addresses of the sender and recipient are correct.
The SMTP connection works and the test mail is send too but no mails are send over a Workflow task or activity.
In the Workflow Engine Error Log you'll see the following error message:
Value cannot be null.
Parameter name: address
Solution:
The error appears because the Workflow Engine Server doesn't have the SMTP connection assignment.
Navigate to the DW Administration - Server - Workflow Engine and assign the created SMTP connection.