Mailer should verify settings on startup

[taylortom]

For reference, from http://nodemailer.com/smtp

You can verify your SMTP configuration with verify(callback) call (also works as a Promise). If it returns an error, then something is not correct, otherwise the server is ready to accept messages.

// verify connection configuration
transporter.verify(function(error, success) {
   if (error) {
        console.log(error);
   } else {
        console.log('Server is ready to take our messages');
   }
});