Earlier quoted context omitted.
Ironically, it seems like half of the emails there are just spam.
It's a free service right now. I'm thinking converting it to a paid one like the one in the article might solve this problem.
You could also do some heuristics like not accepting emails with excessive HTML. Though I'm not sure how much it'd work, since you already require confirmation so presumably the spammers actually have someone authorizing the publication. (I didn't test your flow since I have nothing that warrants posting to say)