When the system emails the contents of the Pre-Sales Contact Form to admins, the email address of the submitter is not included in the text of the email. It is properly inserted into the email's "Reply To" field, but that field is not visible to the admin receiving the email. Unless the admin clicks on "Reply" to initiate a reply to the form's submitter, there is no way to see this information. Many form requests require some research before an actual reply can be made, so an immediate reply is not appropriate. So in order to find out who submitted the request, the admin must initiate a reply, note the address to which the reply will be sent, and then cancel out of the reply.
It has been pointed out by WHMCS Support that altering the "From" email address, as another features request has suggested (Offer the ability to choose the "From" address the Contact form uses) is not possible, as it will cause email delivery issues. Placing a simple line of text containing the "Reply To' email address into the email sent to the admins will resolve this issue.