Actually, graylisting [1] works by exploiting the standard, which says mail servers may temporarily reject incoming messages for a variety of reasons (e.g. disk

Actually, graylisting [1] works by exploiting the standard, which says mail servers may temporarily reject incoming messages for a variety of reasons (e.g. disk full). In these situations, the sending server is required to hold the message and try again, and those servers that don't are the ones that are violating the standard. Moreover, the sending user should never see the temporary failure. This is all outlined in Section 6.1 of RFC5321 [2]. [1] https://en.wikipedia.org/wiki/Greylisting_(email) https://en.wikipedia.org/wiki/Greylisting_(email) [2] https://datatracker.ietf.org/doc/html/rfc5321#section-6.1 https://datatracker.ietf.org/doc/html/rfc5321#section-6.1