Email clients and email service providers don't pass referrer information consistently. Traffic that comes from an email will show up either as a domain from email service providers like gmail, Yahoo or AOL or as "direct" traffic.
Using UTM parameters in your email newsletters or sales campaigns is the best way to pass email referrer data consistently. A best practice is to use "utm_medium=email" as the parameter.
An example URL might look like: https://your-site.com?utm_medium=email&utm_campaign=july_newsletter
In Tend's dashboard, click "edit columns" to expose the UTM parameters you want to see:
Then, use Tend's search advanced to see any visitor or contact that came to your site after clicking a link in an email with those UTM parameters: