How to Set Up DKIM

Prerequisites

Before setting up DKIM, make sure you have:

• A domain name you control
• Postfix email server installed
• Root or sudo privileges
• Access to DNS management for your domain

Step 1: Install OpenDKIM

Connect to your VPS:

ssh hxroot@YOUR_SERVER_IP -p 22

sudo apt install opendkim opendkim-tools -y

Step 2: Create DKIM Keys

sudo mkdir -p /etc/opendkim/keys/example.com
cd /etc/opendkim/keys/example.com
sudo opendkim-genkey -D /etc/opendkim/keys/example.com -d example.com -s mail
sudo chown -R opendkim:opendkim /etc/opendkim/keys
sudo chmod 600 /etc/opendkim/keys/example.com/*

mail is the selector name – you can choose any.

Step 3: Configure OpenDKIM

sudo nano /etc/opendkim.conf

Ensure these lines:

Domain                  example.com
KeyFile                 /etc/opendkim/keys/example.com/mail.private
Selector                mail
Socket                  inet:8891@localhost
UserID                  opendkim

Create or edit /etc/default/opendkim:

RUNDIR=/run/opendkim
SOCKET="inet:8891@localhost"

Step 4: Configure Postfix to Use OpenDKIM

sudo nano /etc/postfix/main.cf

Add:

# DKIM
milter_default_action = accept
milter_protocol = 6
smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891

Step 5: Restart Services

sudo systemctl restart opendkim
sudo systemctl restart postfix

Step 6: Get DKIM Public Key for DNS

sudo cat /etc/opendkim/keys/example.com/mail.txt

Copy the TXT record value (everything after v=DKIM1;).

Step 7: Add DKIM Record in DNS

Create a TXT record for mail._domainkey.example.com with the value from step 6.

Example:

mail._domainkey IN TXT "v=DKIM1; h=sha256; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQD..."

Step 8: Verify DKIM

Test with:

sudo opendkim-testkey -d example.com -s mail -vvv

Send a test email and check headers for DKIM-Signature and Authentication-Results: dkim=pass.