I have a FortiMail with Greylisting turned on. The idea is that only after a valid SMTP retry, an email will get released.
However, it appears that for a large email provider like outlook.com, that SMTP retries arrive from different IP addresses, which completely defeats Greylisting.
Does RFC 2821 specify that SMTP retried MUST come from the same IP address? If not, I don't see how Greylisting can work.