SMTP Program Problem

I'm working at a program that needs to send a mail from a local host.
I know how find the address of target mail server out.
I Know how I establish a connection at port 25 of the mail server.
But, when I try to pass "RCPT TO: <address@provider>" the mail server prevent me going on.
I get a messages like "Invalid network.", "You are not authorized by domain to send email.", and so on.
I guess this have something to do with the Sender Policy Framework.
I have tried first send a mail to me in my provide, but even if I authenticate me (AUTH LOGIN), the transaction fails.
I think this is possible. Mail Clients do it.
Does anybody a suggestion?