Greylisting is a method used to defend email accounts against spam.
When a mail server receives email from an unrecognized sender it temporarily rejects it and informs the sending mail server to resend the email. A legitimate mail server will resend the email after a short delay; the email would then be accepted by the receiving mail server and subjected to further spam checks. The receiving mail server adds the sender to its whitelist for a specified period of time (i.e. 1 month). Subsequent emails from the sender won’t undergo greylisting during this period.
Greylisting works in reducing the amount of spam received because a lot spammers won’t bother to resend the rejected email.