It’s ‘Modern Cassandra’, originally published in F&SF a couple of years ago, now free to read in Small Wonders:

Modern Cassandra tries so hard to warn us, but all her visions go to Junk. I have an important revelation! she writes. I saw you in my dreams. This is urgent. Please reply, but even though she flags her emails as top priority we never do.

With thanks to Medium Amanda, who made a really dedicated effort to connect with me towards the end of 2019! And yet I never replied to her. What would have happened if I had?