Greylisting is a term used to defend or to block spam at the sender server. It blocks the spam emails based on the behaviour of the sender server instead of content and images of the email campaign. This term is generally used by URIBL and they maintain a list of greylists, whitelists and blacklists.
Procedure to get into a greylist:
URIBL say the following on their website - "Greylist is a list which contains domains found in UBE/UCE, and possibly honour opt-out requests. It may include ESPs which allow customers to import their recipient lists and may have no control over the subscription methods. This list can and probably will cause False Positives depending on your definition of UBE/UCE. This zone rebuilds several times a day as necessary."
To explain this, a domain is being watched constantly for deliverability issues and when a message is being reviewed by SpamAssassin a greylist appears deliberating false positive scores.
Does Greylists affect my messages getting filtered?
Certainly no, greylists will just give out false positive score while reviewing a message, other than that, it would not raise any issue to stop your emails getting delivered. Many of the domains will appear in a Greylist without any issues.
Want to have a trail run for your campaign? Do you want to test the quality of the service we offer? One of our customer advisers will call you to discuss about your requirement and set up a free trail account for you.