Without setting "defaultCredentials" to true, you can only send email in the same domain. When you send email outside the domain, you will receive "Mailbox unavailable. The server response was: 5.7.1 Unable to relay for whoever@notmycompany.com" error.
You can’t change a person, but you can change the way you see him/her.
You may not change the person, but it’ll change your attitude toward him/her. Who knows? Maybe he/she’ll start to change when you start to see him/her differently!