FYI, the developer has a writeup about the design behind this project: https://medium.com/@fabianterh/how-i-built-heimdall-an-open-...

At first I was confused and trying to figure out what AWS services Heimdall uses to work, and this was the section that explained it:

  Infrastructure
  
  I’m using AWS’s Simple Email Service (SES) to send and receive emails, S3 for storage, and Lambda functions for serverless computing. Here’s how it works:
  
  All received emails trigger SES to store the email as a file in a S3 bucket, which triggers a Lambda function. Depending on the email, one of several things could happen:
  
  1. The email gets forwarded to your personal email address
  2. The email gets forwarded to the original sender (when you reply)
  3. A command is invoked by you (e.g. to generate a new alias)
  4. Nothing happens (when someone emails an invalid/disabled alias)
  
  I chose to use AWS for practical reasons: I’m totally new to cloud computing, and AWS being the most popular cloud computing service means it is easier to find guides and resources online.