Greylisting is a simple but highly effective means to weed out
messages that are being delivered via spamware/ratware tools. The
idea is to establish whether a prior relationship exists between the
sender and the receiver of a message. Most of the time it does, and
the delivery proceeds normally.

On the other hand, if no prior relationship exists, the delivery is
temporarily rejected. Legitimate MTAs will treat this response
accordingly, and retry the delivery in a while. In contrast, ratware
will usually fail to retry the delivery in a normal fashion.