Thanks for your answers.
I think i melt every aspect of this issue.
I'm not good at postfix configuration, and mail servers configuration in général, but i swear, i'm trying to improve
So, first thing first : i have a google account, on which i setup an email address (only to send emails, not receive some) from my own domain (let's say for example : olivier.fr).
So, in the gmail account, i setup en external SMTP server : Server = smtp.olivier.fr (which leads fine to my server's ip), user and password as a linux user i created on the server who can send mail.
But, when i try to send mail from this address, i get an error on gmail :
Unspecified error (SENT_SECOND_EHLO) : Smtp server does not advertise AUTH capability.
What would be the correct configuration, in postfix, to allow Gmail to connect to port 25, and send mails through my server ?
Thanks for this first step, if you're still in after that, 'ill ask you a LOT more questions