Using DKIM

DKIM is an open standard for email authentication used to check the integrity of the content of an email. In this tutorial, we will set up DKIM by:

  • Adding DNS TXT records for DKIM.
  • Generate keys to encrypt and verify emails.
  • Add filtering for incoming and outgoing emails using DKIM.

We will use the domain for this example, but feel free to replace it by your own domain.

Configure the DNS

A new DNS record is added into the DNS zone. This record declares the public key usable to verify the messages. (See the What is DKIM chapter for more details)

TODO: add command line example.

Generate Keys

TODO: add commands using lets encrypt.

vSMTP root configuration

The path to private keys for DKIM can be specified in the /etc/vsmtp/conf.d/config.vsl script:

fn on_config(config) {
  config.server.dkim.private_key = ["/path/to/private-key-1", "/path/to/private-key-2", ...];

Configuring DKIM keys

It is also possible to configure keys per domain.

fn on_domain_config(config) {
  config.dkim.private_key = ["/path/to/private-key-1", "/path/to/private-key-2", ...];

Configuring DKIM keys for a specific domain (f.e.

If a key cannot be found for a specific domain, the root dkim keys are used instead.

Add signatures

Sign an email using the dkim::sign function for outgoing emails.

  postq: [
    action "sign dkim" || {
      // Iterate over all the private keys defined for the server ''

      for key in dkim::get_private_keys("") {
          // Selector of the DNS record.
          // The private key associated with the public key in `{selector}._domainkey.{sdid}`
          // Or `` in that case
          // Headers to sign with.
          ["From", "To", "Date", "Subject", "From"],
          // Canonicalization algorithm to use.


Verify signatures

Verify DKIM signatures of incoming emails by calling the dkim::verify function.

  // ... previous rules ...

  postq: [
    rule "verify DKIM signatures" || {
      if dkim::verify().status == "pass" {
      } else {


To check if DKIM is working correctly, check out this site.