Hi,
I have only half an answer for you.
The default mail address being used for sending a test mail belongs to the Joomla user that initially setup Acymailing. When you change the mail address of this Joomla user Acymailing will pickup the change.
I have no idear where to change the actual Joomla user that is being used for this. I think this is a feature that has not been added to v6 yet. Thus leaving you with the changing the mail address of the Joomla user or to add or change the mail address manually when sending a test mail.