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Issue: Email providers the like of Office365/Outlook.com, SendGrid and Google are using different SMTP servers each time they attempt to send a message. The result is that the email is delayed for longer than originally set in the SM greylisting settings. We have tracked examples with the above providers where a message has been delayed up to 24 hours as a result of this.
Proposed solution: On the attempt after the expiration of the delay. If the IP/Sender combination isn't the same as the initial one, then check SPF/Sender combination and if SPF passes allow the email. If no SPF then stick to the IP/Sender combination.
MDaemon can do this and we have seen the speed of delivery increase from providers that use different SMTP servers to send without a change to the amount of spam getting through.