I've now heard from a couple of angry folks whose inboxes got flooded last night with several hundred emails - notifications of New Post when in reality the posts were old.
My blog used to be hosted here on WordPress.com (as custom domain powerpivotpro.com) but then I transferred it to self-hosted a few years back. I set PowerPivotPro.Wordpress.com to be private about a year ago so people would stop finding the stale content.
My own personal email subscription is via Feedburner, and I received no mails. The people reporting trouble say that their email flood came from [email redacted]
I also have my self-hosted blog connected via Jetpack, AND recently disconnected from Jetpack and re-connected, in case that is relevant.
I've posted to my readers here, apologizing to those affected and asking for more info:
The blog I need help with is www.powerpivotpro.com.