Postmaster is a tiny SMTP server that stores all messages in memory and exposes an HTTP API to retrieve them later. This makes it really easy to test the headers and the content of your emails.
Install Postmaster with npm:
$ npm install -g postmaster
You can then run Postmaster with the postmaster command, which should be in your path:
$ postmaster
> Postmaster reporting for duty on localhost:5666 (HTTP - localhost:5667)
Change the bound hostname and port with the -l and -p flags, respectively:
$ postmaster -p 4432 -l 127.0.0.1
> Postmaster reporting for duty on 127.0.0.1:4432 (HTTP - 127.0.0.1:4433)
The HTTP API will open up on the next port from the Postmaster port. Sending an HTTP GET request to /emails will return a JSON array of the parsed email and associated headers.
$ curl http://localhost:5667/emails