"Nolisting" is a technique whereby a domain uses a primary MX record for a server that doesn't answer. But why do that?
The answer is simple: Some people have conducted tests that show spammers only make one attempt to deliver an email message and when that attempt fails they simply drop the message and move on to the next email message in their queue.
Because properly operating SMTP servers will make multiple attempts to deliver email, legitimate mail should still arrive to your inbox even if there is somewhat of a delay. When a sending server needs to send a message it gathers the MX records for the recipient domain. It then tries the first MX server based on it's priority level per the DNS records. If it fails then the sending server is supposed to try the second MX server, which should be a legitimate working SMTP server.
Sounds pretty simple. But does it work? Check out the details over here. And do a search on "Nolisting" to see what other people have to say about it. Seems as though some mail transfer agents (MTAs) don't behave properly so in some cases you could very well lose email that you want to receive because the senders' mail systems aren't handling their outgoing mail properly.