Stop spam from eamail addresses at mail.ru coming through my contact form

I have honeypot installed, and it does an excellent job with one significant exception. I get dozens of contact form submissions daily all listing mail.ru email addresses. I’ve blocked the domain at the cpanel level, but that doesn’t solve the problem because the contact form messages are sent from [email protected]. I just want to stop all contact form submissions where the user has listed a mail.ru email address. I understand that there may be legitimate uses of that domain, but I’m comfortable making the assumption that all the messages coming to me from that domain are spam.

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/22/feed