@mac.com email addresses no longer work? Cannot verify/send from

I’ve had a shortened “alias” version of my email address for years, since Apple used to allow you to create several addresses that would all connect to your main ID. It’s from back when things were @ “mac.com”. I’m suddenly not able to send any emails from it! I still receive emails, but when I do a reply and select that email address, it defaults to the horrible “@me.com” address that’s in there. I’m unable to verify it, either, when I attempt to add it as a way I can be reached. Does anyone know what’s going on?