I see that he speaks portuguese, so I can maybe help.
Joilson,
O que ele quis dizer eh que seu servidor tem que ter o hostname correto (tem que bater com o servico de email) e tbm tem que ter um DNS Reverso, que eh o contrario de um DNS normal, ou seja, ao inves de resolver o nome do dominio pro IP, resolve do IP pro nome de dominio. Segue o tutorial no link que ele passou.
Translation:
Joison,
What he means is that your server has to have the correct hostname, and also a reverse DNS, ie. instead of resolving your IP from your Domain, it resolves Domain from IP. Please follow the guide he quoted.