Emails are checked upon arrival and will display a green tick badge if the email is valid. ✅
The badge shows that the email is genuine and from the sender's email address. Some spam emails may show the validation badge as the badge simply shows the email came from that sender.

The validation badge is enabled in the email appearance section of the app preferences. View

The validation process will use information about the email such as SPF, DKIM and DMARC. If these validation checks are valid then the green badge will be visible for the email in the email list screen.
Emails without the validation badge may still be valid but the sender's email server does not supply details that can be used to validate the email.