I have been a follower of justleak.it. Would you run into the same issues as them?
A problem you may run into, if your system takes off and attracts many users, is abuses which result in you being black-listed by anti-spam databases like SORBS, SpamCop and Barracuda. I suspect it will be hard to get de-listed, based on the service that you operate.
Since black-listing is by raw IP address, you might make your hosting provider upset. Suppose linode.com dynamically reshuffles a pool of IP addresses among users. You could get some IP addresses black-listed, and then those IP addresses could be cycled to another user under members.linode.com, who then has a hard time sending mail because of you.
Just a thought.