Greylisting is a protection mechanism against spam. With this gray list used in mail servers, e-mails with unknown e-mail addresses are sent back to the sender with the indication that they will be sent again later. After resending, the e-mail is accepted.
The greylist technique assumes that spam is sent only once and that a spam server does not respond to returned spam. In the case of smart cards, the greylist is a list of those smart card numbers that are under observation, for example because they have already committed an offense against the smart card terms and conditions.
In addition to the greylist, there is also the whitelist, which is a positive list, and the blacklist, a negative list in which persons, systems or websites are listed that are excluded from certain services.