A first-time sender gets a temporary rejection from a server it has never talked to before, and the message is delayed by minutes. That is greylisting, and it is the single most misdiagnosed response in transactional email. What the server is doing It records the triplet of sender IP, envelope ...
Source: [Dev.to](https://dev.to/volanea/greylisting-is-a-4xx-you-should-not-panic-about-1bmi)