Greylisting is a means of automated spam blocking.
It works based on the fact that software designed for sending spam behaves differently than ordinary email servers. If the recipient’s mail server does not accept the message and responds with a “temporary error”, then the sending server should try again later. Spamming programs, however, are usually unwilling to do so.
For servers (or large SMTP servers) special, more gentle greylisting rules are applied. This increases the overall speed by which your mailings are delivered to their recipients.