postfix/smtpd: warning: connect to Milter service unix:/var/run/opendkim/opendkim.sock: No such file or directory

1. Check if opendkim is running. (I assume it is as you saw the socket file.)

2. Did you configure opendkim? The configuration file is /etc/opendkim.conf.

   You need to update the file to match your site/domain and dkim.key path.

3. Add postfix to opendkim group

   If opendkim.sock permission is as follow

   $ ls -l /var/run/opendkim
   
   -rw-rw-r-- 1 opendkim opendkim 6 May  2 14:56 opendkim.pid
   srwxrwxr-x 1 opendkim opendkim 0 May  2 14:56 opendkim.sock
   

   If Not, make sure UMask is set to 0002 in /etc/opendkim.conf.

   Then do the following

   sudo adduser postfix opendkim

4. Postfix running in chroot

   Modify /etc/default/opendkim, change SOCKET option to postfix chroot location

   SOCKET="local:/var/spool/postfix/var/run/opendkim/opendkim.sock"

   You will have to create directory /var/spool/postfix/var/run/opendkim and change its permission

   sudo mkdir -p /var/spool/postfix/var/run/opendkim
   sudo chown opendkim:opendkim /var/spool/postfix/var/run/opendkim
   

5. Restart opendkim

   sudo service opendkim restart
   

Quick summary that helped me to fix this warning on Ubuntu 16.04 LTS: Postfix version 3.1.0-3ubuntu0.2 Opendkim version 2.10.3-3build1

• (I assume that Postfix and Opendkim are already installed, integrated with each other and the only warning you are getting is 'connect to Milter service local:/var/spool/postfix/opendkim/opendkim.sock: No such file or directory')

• Add user postfix to group opendkim (skip if already done)

sudo adduser postfix opendkim

• Create directory and set owner (skip if already done)

sudo mkdir -p /var/spool/postfix/var/run/opendkim

sudo chown opendkim:opendkim /var/spool/postfix/var/run/opendkim

• Check permissions:

ls -l /var/spool/postfix/var/run/opendkim

srwxrwxr-x 1 opendkim opendkim 0 Nov 12 14:34 opendkim.sock

• Edit opendkim configuration files: /etc/opendkim.conf, add line

Socket local:/var/spool/postfix/var/run/opendkim/opendkim.sock

/etc/defaults/opendkim, add

SOCKET="local:/var/spool/postfix/var/run/opendkim/opendkim.sock"

• Edit postfix configuration: /etc/postfix/main.cf

# Milter configuration
# Postfix ≥ 2.6 milter_protocol = 6, Postfix ≤ 2.5 milter_protocol = 2
milter_protocol = 6
milter_default_action = accept
smtpd_milters = unix:var/run/opendkim/opendkim.sock
non_smtpd_milters = unix:var/run/opendkim/opendkim.sock

• Restart services (or reboot)

systemctl restart opendkim

systemctl restart postfix

That's it. I don't know why it's necessary to write

smtpd_milters = unix:var/run/opendkim/opendkim.sock

instead of

smtpd_milters = local:/var/spool/postfix/var/run/opendkim/opendkim.sock

. If somebody knows, explanations are welcome.

I was facing the same issue on Debian stretch, which turned out to be caused by a broken systemd service file for opendkim. See this answer for a solution: https://serverfault.com/a/847442/84962

This fix boils down to:

/lib/opendkim/opendkim.service.generate
systemctl daemon-reload
service opendkim restart