Many email verification and lead generation services act a lot like snowshoe spammers. They bounce around from host to host, hoping to avoid detection and suspicion caused by their unusual SMTP traffic.
Clickback MAIL has been hitting spam trap with email addresses which have been dead for a very long time (i.e. years) or never valid in any decade. Probably because Clickback doesn’t require pesky things like “Opt-In” or “Confirmed” subscribers but do allow purchased lists and “cold” contacts to be mailed. Seriously this is bad behavior, I wonder why no one has just blacklisted all of Clickback IPs/domains yet?
Some spammers have been abusing the University of Minnesota OIT email servers for months, up to dozens of attempts per day. Abuse@ and postmaster@ were notified with overwhelming evidence months ago but the spam continues to trickle through.