The Watch for reply feature is a new button available in the toolbar of the New message window. If you expect a reply for an important email you can set up a special reminder in eM Client.

You can either get a reminder for when such a reply is received to draw your attention to it or you can get reminded after a specific time frame that your message was not replied to, so you can reach out to the recipient again.

After you enable watching for reply a copy of the message will be saved in a special Favorites folder called Unreplied, so you can check which replies you're waiting for at any time.
You can also disable the reminder from this folder, should the recipient reach out to you via other means and the reminder is no longer needed.

This feature is available only to the Pro license users.