Greylisting is a term used by services such as URIBL to designate aspects of sender reputation and overall deliverability. Currently, URIBL maintains a set of safelists, blocklists and greylists.
How Do You Get Added To A Greylist?
According to URIBL's website, a greylist is defined as:
"This lists 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 simplify, this means that a domain is currently being watched for potential deliverability issues. When a service such as SpamAssassin reviews a message, a greylist will likely cause False Positive scores.
When My Domain Is On A Greylist, Will My Messages Be Filtered?
There are no negative effects from being greylisted. Though this can cause false positive scores when reviewing the message, your messages will still be sent out without issue. Many common domains will appear on URIBL greylists regularly.