Greylisting is a new anti-spam implementation offered by cPanel/WHM. It works on the principle that SMTP servers should be configured a very particular way and that spammers will often use misconfigured servers. By forcing SMTP servers to act a specific way, we can often prevent spam from reaching the server before we need to process or scan it.
How does it work?
When an e-mail is received from a server we haven't seen before, our servers will respond with a status that effectively says "I'm busy right now, please try again shortly.". A correctly configured SMTP server will detect this response and queue the mail to be sent at a later time. Spammers on the other hand usually work on the principle that they must send as much mail as they can as quickly as they can, before they get shut down, so do not honour this response. Instead they will usually give up and not try to send the mail again.
So greylisting will cause e-mail delays?
In some cases, yes, Greylisting will cause delays while we wait for the servers to re-send the e-mail. These delays can be anywhere from 10 minutes to approximately 30 minutes - depending on when the sending SMTP server decides to re-attempt mail delivery. Please note that this only happens in situations where we receive e-mail from a relatively unknown sender. Larger senders, such as well known banks or other e-mail providers are whitelisted by default and will not experience the effects of Greylisting.
I cannot afford to have e-mail delays. Can I disable Greylisting?
Absolutely. If you are not happy with Greylisting and wish to disable this option, you can do so from within cPanel under the 'E-mail' section. Click on 'Greylisting' and then tick the 'Off' button.