The largest subnet Exchange Online will allow whitelisted in the connec

The largest subnet Exchange Online will allow whitelisted in the connection filter is /24. This is a huge list. What are other options? Can our emails be narrowed down to a specific range?

I understand but when the form is submitted, emails can be sent using any of the server with different IP addresses. It is not possible to ensure that emails are being sent only from a specific IP range. 

Please note that you can also use your own email address with SMTP setting to send submission emails. The following guide should help you how to setup SMTP in form: https://www.jotform.com/help/244-How-to-Setup-SMTP-for-a-Form

Hope this helps.

Do get back to us if you have any questions.