MailChannels allows free email sending using a Cloudflare Worker. This code adapts the example provided in the article to implement DKIM signing. Official documentation on how to sign messages with DKIM is available here.

Before you start, make sure to set up Wrangler as explained here.

We'll need to generate keys and a DNS record, which we can do with the steps in this tutorial.

To generate a private key and upload it to the worker, run the following command:

openssl genrsa 2048 | tee priv_key.pem | openssl rsa -outform der | openssl base64 -A | wrangler secret put DKIM_PRIVATE_KEY

The parts of this command:

openssl genrsa 2048 generates a 2048-bit RSA key. The output is passed to

tee priv_key.pem, which writes the key to the priv_key.pem file, and passes the key to

openssl rsa -outform der | openssl base 64 -A, which converts the key from PEM format to DER format, then base64 encodes it (this essentially removes the header from the PEM formatted key). The output is finally passed to

wrangler secret put DKIM_PRIVATE_KEY, which adds it as a secret to our Cloudflare Worker.

Now, we need to add the public key to a TXT record. Run the following command:

echo -n "v=DKIM1;p=" > record.txt && openssl rsa -in priv_key.pem -pubout -outform der | openssl base64 -A >> record.txt

This creates a public key from the private key (openssl rsa -in priv_key.pem -pubout -outform der), encodes it in base64 (openssl base 64 -A), and finally writes it to the record.txt file.

Copy the contents of the record.txt file and add it as a TXT record to your domain.

Find these lines in src/index.ts:

            to: [
              {
                email: "", // add your to email here
              },
            ],

            dkim_domain: "", // add your domain

        from: {
          email: "", // add your from email here
        },

and add in your to and from email addresses, as well as the domain you are sending from.

Run wrangler dev or wrangler publish, and you should be able to test it out!