add from field to nodemailer mailOptions

On some smtp servers, the `from` field is mandatory. For example, on Hotmail `smtp.live.com`, when using the plugin without the form field the following error comes back from the server :
    
    { [SenderError: Mail from command failed - 501 5.5.4 Invalid Email address] name: 'SenderError', data: '501 5.5.4 Invalid Email address' }

After adding the field, the email is sent successfully.