Since a form can contain multiple, or even no email addresses, the best way to reply to a message you receive via a form submission is to copy-and-paste the appropriate email address and start a new message using your preferred email client. This will allow you to hold a conversation as needed using a standard email account, rather than going through the website.

Also, a copy of that reply is not stored on the website, but a history of the submitted form will be saved to that page.