Email sent from my iCloud email has started going to everyone’s Junk

I have a username@me.com email address that I've had since around 2011. All of a sudden, about two weeks ago, I've stopped getting replies from people.

Turns out that my emails—including responses to an already-ongoing thread—are ending up in people’s SPAM and Junk folders.

Why is this happening and how can I stop it? It's a huge problem.