@dan As I said, I used SWAKS to confirm that Postfix work well.
The issue is that sometimes the messages are sent, sometimes not.
I am not interested to test with Mailcatcher as is a fake SMTP server used only for testing purposes.
Bottom line, I think FusionAuth is a great CIAM product.
Maybe, by installing by RPM package on a RedHat , we will not have this issue.
Thank you for your support.