Mike is right, @lxoliva. It's the WP ActivityPub plugin. From an ActivityPub-enabled service, subscribe to "@email@example.com".
so there's some incompatibility on either (or both?) ends
gnusocial.net has activitypub support
that's how I get to follow mastodon accounts, after mastodon dropped statusnet support
are other AP implementations able to follow WP AP instances?