Mail getting stopped by Greylisting?

When a sending mail server gets a retry order (in this case a 451 grey-listing response) they should requeue and resend their mail at a reasonable time frame.

The published Internet Standard for retrying to send mail is 30 minutes after the first delivery failure.

http://www.rfc.net/rfc2821.html

“…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…Experience suggests that failures are typically transient (the target system or its connection has crashed), favoring a policy of two connection attempts in the first hour the message is in the queue, and then backing off to one every two or three hours.”

Sometimes the sender is using a Microsoft Exchange Server which has a documented problem with this standard.  Microsoft documents the issue and provides a Registry fix

http://technet.microsoft.com/en-us/library/aa998772.aspx

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