Look at your campaign reports. If you sent a single email from the contact record, you won't see status, only from campaigns. That's why we recommend always creating a campaign, even if the "campaign" is just a single message. You can send a campaign to one user at a time, you don't need to always send it to a group of individuals. Lastly, it's always best practice to include a trackable link in your email ... that way you can tell if they click on your link.