We at junkemailfilter.com are offering a free MX backup service. If your email server goes down we will store and forward your email. And you will see some overall spam reduction from spam bots that try to go through the back door.
What are we doing this? It helps build our visibility and it helps build our spam bot blacklist so that we can block more spam for our spam filtering customers. And because we get some free services we need to give some back. We also have a free blacklist of mostly spam bots that is available for use.