SmtpJS completely securing credentials?

Question

I am trying to send an email using my smtp server and SmtpJS. The problem is that even when using a secureToken generated on their website someone can just go to the sources, modify the destination email and practically send an email to someone pretending it's me. How can I set a permanent email address that my server can send emails to? Hope I'm clear enough. Here is the example from smtpjs.com

Email.send({
    SecureToken : "C973D7AD-F097-4B95-91F4-40ABC5567812",
    To : '[email protected]',
    From : "[email protected]",
    Subject : "This is the subject",
    Body : "And this is the body"
}).then(
  message => alert(message)
);