On 07/25/2017 01:19 PM, Rafael Mora wrote:
What should I do to tell Postfix to use my domain?? for example send emails using the email account firstname.lastname@example.org
If what you're asking is how to configure Postfix to relay all outbound mail via some other email server, that is a Postfix configuration question.
If I thought I could give you a simple answer and that would be the end of it, I'd try, but in your case every answer leads to another question, so I suggest you start here to learn about configuring Postfix http://www.postfix.org/documentation.html.