Greylisting defends email users against spam. When enabled, the mail server will temporarily reject any email from a sender the server does not recognize. If the email is legitimate, the originating server will try again after a delay. After sufficient time has elapsed, the server will accept the email.
Trusted Hosts
Greylisting will never defer emails from entries on the Trusted Hosts list.
Common Mail Providers
Greylisting will never defer emails from trusted mail providers.