Hi Sidney, I was online with Adobe Chat yesterday and they had to refer it to their "level 2", not sure what that is but they were finally able to resolve the issue early this morning. This is a portion of the email I received from them, stating what they needed to do:
It appears that the issue here was related to the fact that you did not have the non-www record for the domain xxxxxx.com. I have since added xxxxxx.com to forward to the default hostname, which is www. xxxxxx.com. Once the non-www record was re-added, it was then possible to navigate to Site Settings > E-Mail Accounts and view the current email users and thus add firstname.lastname@example.org as a mail user.
Thanks for responding, Sidney.