Simple email relay built with node.js and mailgun

index.js

const addrs = require('email-addresses')
const RFC2821 = require('address-rfc2821')
const formBody = require('body/form')
const multiparty = require('multiparty')
const pify = require('pify')
const send = require('@polka/send-type')

/* global apiKey domain read send_email */

const mailgun = require('mailgun-js')({
	apiKey, 
	domain
})

const messages = mailgun.messages()

// https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme
// https://github.com/jichu4n/srslib/blob/master/srslib.py - there are no JS SRS libraries that work.
function srs_reverse(address) {
	// SRS1 is not supported on account of it being so rare.  Low priority TODO
	const match = /SRS0[-=+]([^=]+)=([^=]+)=([^=]+)=(.+)/.exec(address)
	if(!match) return address

	const [,,, domain, forwarder] = match
	let { user } = new RFC2821.Address(forwarder)
	return `${user}@${domain}`
}

function mailgun_webhook(req, res) {
	if(req.headers['content-type'] == 'application/x-www-form-urlencoded') {
		req.body = await pify(formBody)(req, res)
	} else {
		let form = new multiparty.Form()
		// ignore files
		let { err, fields } = await new Promise(r => form.parse(req, (err, fields) => r({ err, fields })))
		if(err) return send(res, 500, err)
		for(let key in fields) {
			if(fields[key].length == 1) {
				fields[key] = fields[key][0]
			}
		}
		req.body = fields
	}
	if(!mailgun.validateWebhook(req.body.timestamp, req.body.token, req.body.signature)) {
		return send(res, 403)
	}

	let sender = srs_reverse(req.body.sender)

	let recipient_relay
	try {
		// fetch user's true email from the database
		recipient_relay = await read('relay', req.body.recipient.replace(`@${domain}`, ''))
	} catch(e) {
		// just a convenience function
		await send_email({
			to: sender,
			template: 'no_such_email'
		})
		return
	}

	// ensure that the sender's email has a matching system address
	let sender_relay = await ensure_relay(sender)

	await messages.send({
		from: `${addrs.parseOneAddress(req.body.from).name} <${sender_relay._key}@${domain}>`,
		to: recipient_relay.email,
		subject: req.body.subject,
		html: req.body['body-html'],
		text: req.body['body-plain']
	})
}