When an email is sent to the Lead record, the Update Client Details app creates a task which is also added to the activity history of the leads, as shown in the image below.
Additionally, there are the managed fields of the Update Client Details app on the Lead object, which get updated when an email is sent to the Lead record, as shown below -
This helps you to keep track of when the Lead record have received the form previously and by which e-mail.
You can add these fields into your lead layout for better understanding.