What is the Sender Email?

I believe you are referring to the Sender Email field of Notification and Autoresponder emails.

This is the field what the users can see when they receive Notification/Autoresponder emails. All the Notifications and Autoresponder will be sent by with this email address. We have the noreply@jotform.com as the Sender Email. 

If you wish to use custom Sender Name and Email address, you can use verified or SMTP emails.

Here are the guides: How-to-Setup-SMTP-for-a-Form

How-to-Add-a-Custom-Sender-Address-to-an-Email-Alert