Where does the form pick up the sender’s email address from?

Thanks Welvin,
Where does the form pick up the sender’s email address from?
When I click on the dropdown list, all I get is the list of fields in my form. How can I ensure the person who sends the form is in the reply-to-Email box?
Thanks
John
...

You should select the email field. I've checked your form and you have the Customer Email field in the settings. The submitted email address in the field will be your Reply-To Email once received. You may consider testing that out. 

The customer is not the sender.  It's one of our sales people who fills out the form together with all other documentation, so I want the person who completes the form, i.e. our sales person (the one shown in the 'Survey Completed by' field) to be the one who's email address is used.  The only way I can see that happening is to add the salesman's email under their name in the form.  Am I right in that?

Hello,

You can add a field in the form such as a dropdown with the list of email addresses or a field which your sales person can enter his email address, and then set that field as your point of source for your reply-to email in your email notification settings.  This way, once the form is submitted, the email address set from that field will be the email address to be used.

Here's a sample of the field added in the form and using it as a field for reply-to email source:

If you have questions, please let us know.

Thanks.