Hi Matt
This is because Factile uses Gmail's email server (SMTP) to send emails and Gmail does not allow custom "from" addresses (so that people/ software do not abuse somebody else's account). This seems to be the case with most of the available email services online.
However, the emails have a reply-to set to the custom email that you specify, so if the users do a reply, that would come to your specified account.
Also, if you would like to install Factile on your servers and leverage your company's mail servers, I can supply the steps and help you do that
Best regards
Aishwarya