Unable to send mail to my personnal domain

Not sure it is the good place for that…
I am working on permit users to subscribe by mail to my personnal ghost website.
When the subsciber mail is like user@free.fr a mail is sent by my ghost website
When the subsciber mail is like user@mydomain.eu no mail is sent so that the user is unable to subscibe

I specify that I use postfix as the local mail server. If I try to send mail directly from the shell (with mutt), mails are sent to free.fr and mydomain.eu
I can check if mail is sent to postfix process in log fail /var/log/mail.log
Is there a Ghost log file I can search for messages ?

Thanks in advance