I respectfully disagree. Subscription model is the only one invented so far that allows to keep any service afloat. Devs constantly input resources - be it fiat for paying for the servers or own time to improve the filter/relay performance. As spammers and attackers get better so must the relays.

Free services like google and Facebook are constantly selling your data, that is their way to fund their business, so paying 3 bucks a month for spam filter and ddos fighter is very reasonable imho.