I'm assuming that you are sending mail through SparkPost and you're accepting email transmission events via webhook from SparkPost.

The simplest way to implement and scale this would be to use Amazon's AWS API gateway connected to their AWS Lambda event-driven architecture. The architecture would look like:

SparkPost -> AWS API gateway -> AWS lambda -> external services

You can define webhook API's in the AWS API gateway very easily, with a GUI interface. The AWS lambda event driven code can be written in NodeJS (JavaScript).

This will scale to arbitrary levels of events without requiring you to invest in a lot of infrastructure.

See these examples:
https://developers.exlibrisgroup.com/blog/Hosting-a-Webhook-Listener-in-AWS

http://docs.aws.amazon.com/apigateway/latest/developerguide/getting-started.html

For a startup, I'd suggest at a minimum you use a CDN like Fastly or Cloudflare in front of your application to prevent basic DDoS attacks. In addition, you'll need to pay particular attention to rate-limiting your login and registration APIs/forms so that you won't have bot attacks and brute-force attacks against your site. The CDN's usually don't provide rate-limiting so you may want to use something like Amazon AWS's functionality for this.

Distil Networks is one of the leaders in this space. They have a good product, although it does add one more layer of complexity to your stack (especially if you already use a CDN). I'd suggest you look at Fastly's offering - they have partnered up with PerimeterX to provide Fastly CDN + Fastly WAF + PerimeterX bot detection all in one package.