I'd need FTP access in order to debug this. I'd start by setting a FROM address that uses your domain like: firstname.lastname@example.org
You can use the entered email address as the ReplyTo address. Some servers won't send spoofed emails that appear to come from a server they don't actually come from.
I see you are sending TO a gmail address. They would likely spam block an email that says it is FROM an email address that it isn't. You may have to also make sure your DNS has a SPF record and DKIM is set up on your mail server before gmail will not block it as spam. Gmail and hotmail are notoriously particular with their spam filters.