From Polymail’s features page:

Know exactly when your messages were read and who read them with real-time notifications and activity feed.

In an email update today:

In our latest update, Polymail now detects and blocks email tracking on your incoming messages to prevent senders from knowing when you’ve opened their emails!

This is a double standard in a way, but I see nothing wrong with it. If you don’t want to “spy” on when others read your email, you don’t have to use this feature. And if you don’t want others spying on you, just use Polymail as your email client and enable this prevention feature.