All work OK except when I send a mail from nonexistent-user@my-domain to user@my-domain,
courier permit send, no has control about identity of nonexistent-user that send mail =(.
SMTP doesn't define a mechanism for verifying return addresses. You can
always send an email with whatever "From" address you want, regardless
of the server, unless the server has a "dialback" mechanism of some type
(which is uncommon).