Quite a few ways. If you're reasonably confident the recipient is accepting HTML emails, you can embed something like a 1-pixel image served by a site or resource you control then inspect the site log to check when the image was served. Of course, you can add bells and whistles to have the site or service notify you. If you're reasonably sure the recipient is using what one would call a browser-based email reader, then you could serve them a cookie.