0 Can't send email using Mailgun - The specified string is not in the form required for an e-mail addr 1 John Johnson posted 5 Years Ago Howdy! I'm brand new to Rock and setup mailgun for mail. When I send a test communication, I receive the error below. I ensured that the web hooks and domain are all configured correctly on Mailgun. Any help would be appreciated.The specified string is not in the form required for an e-mail address..