Remember Lycos Mail? No? Then you're probably under 40 years old. What I didn't remember is that it still existed. But while it shall continue to exist, current users enjoying email service from Lycos Mail for free are being asked to leave as of May 15, 2018. It's not quite the same as shutting down the domain overall, but I'm guessing that 90+ percent of their user base was on a free plan, so it does suggest that soon, most mail to lycos.com addresses will start bouncing. You can read more about it here.
It looks like Lycos Mail is hosted by OpenSRS (Tucows), which has an active Return Path-managed ISP Feedback Loop. I'm guessing Lycos got tired of footing the outsourcing bill for free users. If you're an ESP and you start to see lower complaint volume from your OpenSRS feedback loop after May 15th, that could be why.