greylisting: you need to behave
RFC2821, section 18.104.22.168, "Sending Strategy":
"In a typical system, the program that composes a message has some method for requesting immediate attention for a new piece of outgoing mail, while mail that cannot be transmitted immediately MUST be queued and periodically retried by the sender."
"The sender MUST delay retrying a particular destination after one attempt has failed. In general, the retry interval SHOULD be at least 30 minutes; however, more sophisticated and variable strategies will be beneficial when the SMTP client can determine the reason for non-delivery."
RFC2821 goes on to state that
"Retries continue until the message is transmitted or the sender gives up; the give-up time generally needs to be at least 4-5 days."