How to Set up SPF and DKIM with Postfix on Ubuntu Server

Important: make sure to have correct MX, A and PTR records. To check all record are correct use tools like :

SPF and DKIM are two types of TXT records in DNS that allow you to detect email spoofing and help legitimate emails deliver into the recipient’s inbox instead of the spam folder

Step 1: Create an SPF Record in DNS

if you are not sure about how to write these SPF records. I find the site is good easydmarc or

Create a new TXT record at your DNS management deshboard

TXT  @   v=spf1 mx ~all

you can use your public ip4 address which has the correct PTR. (suggested) Else you can still use TXT  @   v=spf1 mx ~all

To check if your SPF record is propagated to the public Internet, you can use the dig utility on your Linux box.

dig txt

The txt option tells dig that we only want to query TXT records.

Step 2: Configuring SPF Policy Agent

Postfix SMTP server to check for SPF records of incoming emails. This doesn’t help ensure outgoing email delivery but helps with detecting forged incoming emails.

First, install the required packages:

sudo apt install postfix-policyd-spf-python

Then edit the Postfix master process configuration file.

sudo nano /etc/postfix/

Add the following lines at the end of the file, which tells Postfix to start the SPF policy daemon when it’s starting itself.

policyd-spf  unix  -       n       n       -       0       spawn
    user=policyd-spf argv=/usr/bin/policyd-spf

Save and close the file. Next, edit Postfix main configuration file.

sudo nano /etc/postfix/

Append the following lines at the end of the file. The first line specifies the Postfix policy agent timeout setting. The following lines will impose a restriction on incoming emails by rejecting unauthorized email and checking SPF record.

policyd-spf_time_limit = 3600
smtpd_recipient_restrictions =
   check_policy_service unix:private/policyd-spf

Save and close the file. Then restart Postfix.

sudo systemctl restart postfix

Next time, when you receive an email from a domain that has an SPF record, you can see the SPF check results in the raw email header. The following header indicates the sender sent the email from an authorized host.

Received-SPF: Pass (sender SPF authorized).

Step 3: Setting up DKIM

First, install OpenDKIM which is an open-source implementation of the DKIM sender authentication system.

sudo apt install opendkim opendkim-tools

Then add postfix user to opendkim group.

sudo gpasswd -a postfix opendkim

Edit OpenDKIM main configuration file.

sudo nano /etc/opendkim.conf

Find the following line.

Syslog               yes

By default,  OpenDKIM logs will be saved in /var/log/mail.log file. Add the following line so OpenDKIM will generate more detailed logs for debugging.

Logwhy               yes

Locate the following lines.

#KeyFile                /etc/dkimkeys/dkim.key
#Selector               2007

By default, they are commented out. Please don’t uncomment them.

Then, find the following lines. Uncomment them and replace simple with relaxed/simple.

#Canonicalization   simple
#Mode               sv
#SubDomains         no

Then add the following lines below #ADSPAction continue line. If your file doesn’t have #ADSPAction continue line, then just add them below SubDomains  no.

AutoRestart         yes
AutoRestartRate     10/1M
Background          yes
DNSTimeout          5
SignatureAlgorithm  rsa-sha256

Next, add the following lines at the end of this file. (Note that On Ubuntu 20.04, the UserID is already set to opendkim)

#OpenDKIM user
# Remember to add user postfix to group opendkim
UserID             opendkim

# Map domains in From addresses to keys used to sign messages
KeyTable           refile:/etc/opendkim/key.table
SigningTable       refile:/etc/opendkim/signing.table

# Hosts to ignore when verifying signatures
ExternalIgnoreList  /etc/opendkim/trusted.hosts

# A set of internal hosts whose mail should be signed
InternalHosts       /etc/opendkim/trusted.hosts

Save and close the file.

Step 4: Create a Signing Table, Key Table and Trusted Hosts File

Create a directory structure for OpenDKIM

sudo mkdir /etc/opendkim

sudo mkdir /etc/opendkim/keys

Change the owner from root to opendkim and make sure only opendkim user can read and write to the keys directory.

sudo chown -R opendkim:opendkim /etc/opendkim

sudo chmod go-rw /etc/opendkim/keys

Create the signing table.

sudo nano /etc/opendkim/signing.table

Add the following two lines to the file. This tells OpenDKIM that if a sender on your server is using a address, then it should be signed with the private key identified by The second line tells that your sub-domains will be signed by the private key as well.


Save and close the file. Then create the key table.

sudo nano /etc/opendkim/key.table

Add the following line, which tells the location of the private key.

Save and close the file. Next, create the trusted hosts file.

sudo nano /etc/opendkim/trusted.hosts

Add the following lines to the newly created file. This tells OpenDKIM that if an email is coming from localhost or from the same domain, then OpenDKIM should only sign the email but not perform DKIM verification on the email.

Save and close the file.

Note: You should not add an asterisk in the domain name like this: * There should be only a dot before the domain name.

Step 5: Generate Private/Public Keypair

Since DKIM is used to sign outgoing messages and verify incoming messages, Generates a private key for signing and a public key for the remote verifier. The public key will be published in DNS.

Create a separate folder for the domain.

sudo mkdir /etc/opendkim/keys/

Generate keys using opendkim-genkey tool.

sudo opendkim-genkey -b 2048 -d -D /etc/opendkim/keys/ -s default -v

Make opendkim as the owner of the private key.

sudo chown opendkim:opendkim /etc/opendkim/keys/

And change the permission, so only the opendkim user has read and write access to the file.

sudo chmod 600 /etc/opendkim/keys/

Step 6: Publish Your Public Key in DNS Records

Display the public key

sudo cat /etc/opendkim/keys/

The string after the p parameter is the public key.

DNS manager, create a TXT record, enter default._domainkey in the name field. Then go back to the terminal window, copy everything between the parentheses and paste it into the value field of the DNS record. You need to delete all double quotes and white spaces in the value field. If you don’t delete them, then the key test in the next step will probably fail.

Step 7: Test DKIM Key

Enter the following command on Ubuntu server to test your key.

sudo opendkim-testkey -d -s default -vvv

If everything is OK, you will see Key OK in the command output.

opendkim-testkey: using default configfile /etc/opendkim.conf
opendkim-testkey: checking key ''
opendkim-testkey: key secure
opendkim-testkey: key OK

Note that your DKIM record may need some time to propagate to the Internet. Depending on the domain registrar you use, your DNS record might be propagated instantly, or it might take up to 24 hours to propagate. You can go to, enter default as the selector and enter your domain name to check DKIM record propagation.


Step 8: Connect Postfix to OpenDKIM

Create a directory to hold the OpenDKIM socket file and allow only opendkim user and postfix group to access it.

sudo mkdir /var/spool/postfix/opendkim

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

Then edit the OpenDKIM main configuration file.

sudo nano /etc/opendkim.conf
(Ubuntu 22.04/20.04)
Socket    local:/run/opendkim/opendkim.sock

Replace it with the following line. (If you can’t find the above line, then add the following line.)

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

Save and close the file.

If you can find the following line in /etc/default/opendkim file.




Change it to


Save and close the file.

Next, edit the Postfix main configuration file.

sudo nano /etc/postfix/

Add the following lines at the end of this file, so Postfix will be able to call OpenDKIM via the milter protocol.

# Milter configuration
milter_default_action = accept
milter_protocol = 6
smtpd_milters = local:opendkim/opendkim.sock
non_smtpd_milters = $smtpd_milters

Save and close the file. Then restart opendkim and postfix service.

sudo systemctl restart opendkim postfix

Step 9: SPF and DKIM Check

send a test email from your mail server to your Gmail account to see if SPF and DKIM checks are passed. On the right side of an opened email message in Gmail, if you click the show original button from the drop-down menu, you can see the authentication results.

Checking the OpenDKIM Logs

Sometimes, the OpenDKIM journal logs may help you find out what’s wrong.

sudo journalctl -eu opendkim


Also check the mail log file: /var/log/mail.log




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