Dkim multiple mail servers

DKIM (DomainKeys Identified Mail) is a technique for authenticating the sender of an email message. When you have multiple mail servers, you'll need to set up DKIM on each server to ensure that all outgoing emails are signed with the same domain-specific key. Here's a step-by-step guide to help you set up DKIM on multiple mail servers:

Prerequisites:

1. You have multiple mail servers (e.g., Postfix, Sendmail, or Exim) that send emails on behalf of your domain.
2. You have a domain name (e.g., example.com) and a DNS provider (e.g., Cloudflare, AWS Route 53, or Google Cloud DNS).
3. You have a DKIM key pair (private and public keys) generated for your domain.

Step 1: Create a DKIM record

Create a TXT record in your DNS provider's control panel with the following format:

_kim.example.com. IN TXT "v=DKIM1; k=rsa; p=<public_key>"

Replace <public_key> with the public key generated for your domain.

Step 2: Configure DKIM on each mail server

On each mail server, you'll need to:

1. Install the DKIM plugin or module (if not already installed).
2. Configure the DKIM plugin/module to use the private key and the domain name.
3. Set the DKIM selector (e.g., _kim) and the DKIM key (e.g., the public key).

Here are some examples for popular mail servers:

• Postfix: postconf -e "milter_default_action = accept" && postconf -e "milter_protocol = 2" && postconf -e "smtpd_milters = inet:localhost:12345" && postconf -e "non_smtpd_milters = inet:localhost:12345"
• Sendmail: m4 -Dm4 -P (in the sendmail.cf file) and add the following lines:

  define(`_kim', `rsa-sha256')
  define(`_kim_selector', `_kim')
  define(`_kim_key', `<public_key>')

• Exim: In the exim.conf file, add the following lines:

  dkim_selector = _kim
  dkim_key = <public_key>

  Step 3: Test DKIM

Use a tool like opendkim-testkey or dkim-filter to test your DKIM setup. This will help you verify that your mail servers are signing emails correctly and that the DKIM records are being propagated correctly.

Step 4: Monitor and troubleshoot

Monitor your mail server logs and DKIM reports to ensure that emails are being signed correctly and that any issues are being reported. Troubleshoot any issues that arise, such as DNS propagation delays or misconfigured mail servers.

By following these steps, you'll be able to set up DKIM on multiple mail servers and ensure that all outgoing emails are signed with the same domain-specific key.