Payment isn't compulsory. You could have a number of mechanisms to clear mailing lists, white-lists would be easiest. Since joining a mailing list should be b

Payment isn't compulsory. You could have a number of mechanisms to clear mailing lists, white-lists would be easiest. Since joining a mailing list should be by user decision, provided the user interface is simple and painless you should be able to include adding to the whitelist as part of that process.