Email domain on intranet?

I want to use a form on our company’s intranet, which is built with WordPress. However, I need to specify an email address with the correct domain name. We don’t have a domain name internally as the site is just called ‘Inside’ with the address?https://inside. What email address should I enter in the ‘From’ field on the ‘Mail’ tab? I’ve tried various possible and conceivable addresses, but nothing works. Is there any way to work around this?