Node.js and PHP are both good. And you can achieve what you want by going on with anyone. But, it's about ease too.
So, Let's discuss them on few points:
Synchronous or Asynchronous - This must be the most dominating factor for deciding. If you want to be blocked until previous operation is over, that means you are looking out for synchronous else vice-versa. Email-sending, in most of cases is kept asynchronous. So, Node.js will be the better if you want to send bulk-mails asynchronously.
Database - PHP will be better with SQL database and Node.js will be better with NoSQL or storing JSON object. Of course, it's not like hard coded, You can use otherwise too.
Performace - I think for your use case Node.js will be better, as it is very well in non-blocking type of applications. You can go through below link as well. hostingadvice.com/blog/comparing-node-js-vs-php-p…
Ease of writing - PHP has easier syntax. Node.js, being JavaScript, as we know, is all about curly-braces and parentheses and then keep counting them. But, if there's front-end also involved in your application then Node.js might help you, by preventing the syntax/context switches between front-end and back-end code
Now, in terms of framework,
Symfony2 is the one I'll go with if I choose PHP - LAMP or LNMP stack
ExpressJs, if I choose Node.js, which is the most commonly used framework for Node.js - MEAN stack