Building the Network You Need with OpenBSD's PF: BSDCan, Ottawa May 6th, 2009
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# Greylisting: See the RFC

greylisting: you need to behave

RFC5321 (Oct 2008, supersedes RFC2821), section 4.5.4.1, "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."

and

> "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."

RFC5321 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."