The default email is
Says who? I think you are misunderstanding something here. The email address is whatever you create within the Email Server of Cloudron.
First you need to setup an email address at Cloudrons Email Server and assign it to a user. Lets say you create an Email address within Cloudron for our user "bob" with the display name "Bob Marley" that goes like "firstname.lastname@example.org". Now you login to SOGo with the credentials email@example.com and the password of bob.
So to be clear: You are in charge of creating firstname.lastname@example.org in the first place before apps like SOGo and other clients come into play.