Comment by mike_hearn

signal.me probably isn't a major domain. These things work on probability. I use Signal (most people don't) and had never heard of this domain until now. It's likely a utility domain that doesn't appear in X messages often, so a single spammer is capable of overwhelming its good traffic in a single campaign. Whereas e.g. google.com has a lot of presence over a long period of time in non-spam messages so it'd take a huge spam campaign to overpower it.