Send email from Cloudflare Worker

index.ts

/*
Also requires DNS setup for domain:

TXT
_mailchannels
v=mc1 cfid=<YOUR_WORKER_DOMAIN_IN_CLOUDFLARE>.workers.dev


Also DKIM:
https://developers.cloudflare.com/pages/platform/functions/plugins/mailchannels/
https://support.mailchannels.com/hc/en-us/articles/7122849237389-Adding-a-DKIM-Signature

And SPF:
https://support.mailchannels.com/hc/en-us/articles/200262610-Set-up-SPF-Records
https://www.mailerlite.com/help/how-to-merge-spf-records

*/

export interface Env {
	EXPECTED_KEY: string;
	FROM_EMAIL: string;
	FROM_NAME: string;
	DKIM_DOMAIN: string;
	DKIM_PRIVATE_KEY: string;
}

export default {
	async fetch(request, env, ctx): Promise<Response> {
		return await handler({ env, request });
	},
};

const handler = async ({ env, request }: { env: Env; request: Request }) => {
	if (!env.EXPECTED_KEY) {
		throw new Error('EXPECTED_KEY not set');
	}
	if (!env.FROM_EMAIL) {
		throw new Error('FROM_EMAIL not set');
	}
	if (!env.FROM_NAME) {
		throw new Error('FROM_NAME not set');
	}
	if (!env.DKIM_DOMAIN) {
		throw new Error('DKIM_DOMAIN not set');
	}
	if (!env.DKIM_PRIVATE_KEY) {
		throw new Error('DKIM_PRIVATE_KEY not set');
	}

	if (request.method !== 'POST') {
		return new Response('Not supported', {
			status: 405,
		});
	}

	const requestUrl = new URL(request.url);
	const apiKey = requestUrl.searchParams.get('key') ?? '';

	if (apiKey !== env.EXPECTED_KEY) {
		return new Response('Wrong API KEY', {
			status: 401,
		});
	}

	let json;
	try {
		json = await request.json();
	} catch {
		return new Response('JSON body is missing or incorrect', {
			status: 400,
		});
	}
	const recipient = {
		email: json.to?.email?.trim() ?? '',
		name: json.to?.name?.trim() ?? '',
	};

	if (!recipient.email) {
		return new Response('Recipient email not provided', {
			status: 400,
		});
	}

	const sender = {
		email: env.FROM_EMAIL,
		name: env.FROM_NAME,
	}
	const mail = {
		subject: json.subject ?? '',
		html: json.content?.html,
		plainText: json.content?.plain,
	};

	const email_request = new Request('https://api.mailchannels.net/tx/v1/send', {
		method: 'POST',
		headers: {
			'content-type': 'application/json',
		},
		body: JSON.stringify({
			personalizations: [
				{
					to: [recipient],
					dkim_domain: env.DKIM_DOMAIN,
					dkim_selector: 'mailchannels',
					dkim_private_key: env.DKIM_PRIVATE_KEY,
				},
			],
			from: sender,
			subject: mail.subject,
			content: buildContent(mail),
		}),
	});

	const res = await fetch(email_request);
	const resJson = await res.json();

	return new Response(`${res.status}\n${res.statusText}\n${JSON.stringify(resJson, null, 2)}`, {
		status: res.status,
	});
};

const buildContent = (mail) => {
	const result: object[] = [];

	if (mail.plainText) {
		result.push({
			type: 'text/plain',
			value: mail.plainText,
		});
	}

	if (mail.html) {
		result.push({
			type: 'text/html',
			value: mail.html,
		});
	}

	return result;
};