Problem with wixUsersBackend.emailUser()

I added some new functionality into my registration process recently where if an error occurs sending the user a validation email then the user is removed from the site (to avoid spam). This seemed to work for one email I tried out however after doing some more testing today even when an email that doesn’t exist is used, emailUser() still finishes successfully so my catch block never runs.

I assumed attempting to email an address that isn’t actually real would be enough to stop emailUser() running properly, so what are the requirements for the catch block to run in emailUser()? Should I simply be using a captcha instead?

This is the snippet from my registration in question, no matter what email I use the .then() always runs:

return wixUsersBackend.emailUser('XXXXXX', reg.user.id, {
    variables: {
        approvalToken: reg.approvalToken
    }
})
.then( ()=> {
    console.log("REG STATUS ", "Wix-users signup successful.");
    return "SUCCESS";
}).catch( (emailErr) => {
    console.log("REG STATUS ", "Error sending email.");
    
    //Remove user account:
    DeleteUser(reg.user.id, 0);
    
    return "EMAIL ERROR";
});