Thanks for your response.
I have understood you but I don't have a mail server.
I have a domain, with 12 computers more or less, one domain controller and external mailserver. It is a small business.
The software that sends the mails is a "database web". I try to explain you. I have document management software and it contains databases. Also, you can see the databases through Internet because it installed in IIS and it is able to send documents of the database, for this, it need install SMTP service.
But when I send a mail with this software IIS send me the error that I told here.
Whatever I don't have Exchange or another mail server. Then What could I do?
Thanks