Okay so I found the mail log here : /usr/local/psa/var/log/maillog
And what I see is, the server try to send mail on gmail smtp, I never asked for that I don't understand :
Sep 16 13:30:13 localhost postfix/smtp[124418]: connect to gmail-smtp-in.l.google.com[2a00:1450:400c:c02::1b]:25: No route...